Patent Publication Number: US-11036551-B2

Title: Durable program execution

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and is a continuation of co-pending U.S. patent application Ser. No. 15/814,861 entitled “Durable Program Execution” and filed Nov. 16, 2017 which is a continuation of U.S. patent application Ser. No. 13/798,152 entitled “Durable Program Execution” and filed Mar. 13, 2013, the entire disclosures of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     A long-running program may have extensive periods of time between events that involve processing. To conserve resources, the program may be required to save its state between events so that the program can be shut down and removed from memory until the next event applicable to the program is received. This may be done, among other reasons, to make room for other programs to execute on the computing resources. To accommodate the requirement to save state, a programmer may write code that saves state to storage in conjunction with processing an event. The programmer may also write code that loads the state from storage and resumes with the appropriate action when a new event is received. This is cumbersome to programmers. 
     The subject matter claimed herein is not limited to embodiments that solve any disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate one exemplary technology area where some embodiments described herein may be practiced. 
     SUMMARY 
     Briefly, aspects of the subject matter described herein relate to durable program execution. In aspects, a mechanism is described that allows a program to be removed from memory when the program is waiting for an asynchronous operation to complete. When a response for the asynchronous operation is received, completion data is stored in a history, the program is re-executed, and the completion data in the history is used to complete the asynchronous operation. The above actions may be repeated until no more asynchronous operations in the history are pending completion. 
     This Summary is provided to briefly identify some aspects of the subject matter that is further described below in the Detailed Description. This Summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. 
     The phrase “subject matter described herein” refers to subject matter described in the Detailed Description unless the context clearly indicates otherwise. The term “aspects” should be read as “at least one aspect.” Identifying aspects of the subject matter described in the Detailed Description is not intended to identify key or essential features of the claimed subject matter. 
     The aspects described above and other aspects of the subject matter described herein are illustrated by way of example and not limited in the accompanying figures in which like reference numerals indicate similar elements and in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram representing an exemplary general-purpose computing environment into which aspects of the subject matter described herein may be incorporated; 
         FIG. 2  is a block diagram that generally represents exemplary components for durable program execution in accordance with aspects of the subject matter described herein; 
         FIGS. 3-4  are flow diagrams that generally represent exemplary actions that may occur to obtain asynchronous operations from a program and responses thereto in accordance with aspects of the subject matter described herein; 
         FIG. 5  is a flow diagram that generally represents exemplary actions that may occur in providing code to an execution environment in accordance with aspects of the subject matter described herein; and 
         FIG. 6  is a flow diagram that generally represents exemplary actions that may occur for a function invoker in accordance with aspects of the subject matter described herein. 
     
    
    
     DETAILED DESCRIPTION 
     Definitions 
     As used herein, the term “includes” and its variants are to be read as open-ended terms that mean “includes, but is not limited to.” The term “or” is to be read as “and/or” unless the context clearly dictates otherwise. The term “based on” is to be read as “based at least in part on.” The terms “one embodiment” and “an embodiment” are to be read as “at least one embodiment.” The term “another embodiment” is to be read as “at least one other embodiment.” 
     As used herein, terms such as “a,” “an,” and “the” are inclusive of one or more of the indicated item or action. In particular, in the claims a reference to an item generally means at least one such item is present and a reference to an action means at least one instance of the action is performed. 
     Sometimes herein the terms “first”, “second”, “third” and so forth may be used. Without additional context, the use of these terms in the claims is not intended to imply an ordering but is rather used for identification purposes. For example, the phrases “first version” and “second version” do not necessarily mean that the first version is the very first version or was created before the second version or even that the first version is requested or operated on before the second version. Rather, these phrases are used to identify different versions. 
     Headings are for convenience only; information on a given topic may be found outside the section whose heading indicates that topic. 
     Other definitions, explicit and implicit, may be included below. 
     Exemplary Operating Environment 
       FIG. 1  illustrates an example of a suitable computing system environment  100  on which aspects of the subject matter described herein may be implemented. The computing system environment  100  is only one example of a suitable computing environment and is not intended to suggest any limitation as to the scope of use or functionality of aspects of the subject matter described herein. Neither should the computing environment  100  be interpreted as having any dependency or requirement relating to any one or combination of components illustrated in the exemplary operating environment  100 . 
     Aspects of the subject matter described herein are operational with numerous other general purpose or special purpose computing system environments or configurations. Examples of well-known computing systems, environments, or configurations that may be suitable for use with aspects of the subject matter described herein comprise personal computers, server computers—whether on bare metal or as virtual machines—, hand-held or laptop devices, multiprocessor systems, microcontroller-based systems, set-top boxes, programmable and non-programmable consumer electronics, network PCs, minicomputers, mainframe computers, personal digital assistants (PDAs), gaming devices, printers, appliances including set-top, media center, or other appliances, automobile-embedded or attached computing devices, other mobile devices, phone devices including cell phones, wireless phones, and wired phones, distributed computing environments that include any of the above systems or devices, and the like. 
     Aspects of the subject matter described herein may be described in the general context of computer-executable instructions, such as program modules, being executed by a computer. Generally, program modules include routines, programs, objects, components, data structures, and so forth, which perform particular tasks or implement particular abstract data types. Aspects of the subject matter described herein may also be practiced in distributed computing environments where tasks are performed by remote processing devices that are linked through a communications network. In a distributed computing environment, program modules may be located in both local and remote computer storage media including memory storage devices. 
     Alternatively, or in addition, the functionality described herein may be performed, at least in part, by one or more hardware logic components. For example, and without limitation, illustrative types of hardware logic components that can be used include Field-programmable Gate Arrays (FPGAs), Program-specific Integrated Circuits (ASICs), Program-specific Standard Products (ASSPs), System-on-a-chip systems (SOCs), Complex Programmable Logic Devices (CPLDs), and the like. 
     With reference to  FIG. 1 , an exemplary system for implementing aspects of the subject matter described herein includes a general-purpose computing device in the form of a computer  110 . A computer may include any electronic device that is capable of executing an instruction. Components of the computer  110  may include a processing unit  120 , a system memory  130 , and one or more system buses (represented by system bus  121 ) that couples various system components including the system memory to the processing unit  120 . The system bus  121  may be any of several types of bus structures including a memory bus or memory controller, a peripheral bus, and a local bus using any of a variety of bus architectures. By way of example, and not limitation, such architectures include Industry Standard Architecture (ISA) bus, Micro Channel Architecture (MCA) bus, Enhanced ISA (EISA) bus, Video Electronics Standards Association (VESA) local bus, Peripheral Component Interconnect (PCI) bus also known as Mezzanine bus, Peripheral Component Interconnect Extended (PCI-X) bus, Advanced Graphics Port (AGP), and PCI express (PCIe). 
     The processing unit  120  may be connected to a hardware security device  122 . The security device  122  may store and be able to generate cryptographic keys that may be used to secure various aspects of the computer  110 . In one embodiment, the security device  122  may comprise a Trusted Platform Module (TPM) chip, TPM Security Device, or the like. 
     The computer  110  typically includes a variety of computer-readable media. Computer-readable media can be any available media that can be accessed by the computer  110  and includes both volatile and nonvolatile media, and removable and non-removable media. By way of example, and not limitation, computer-readable media may comprise computer storage media and communication media. 
     Computer storage media includes both volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information such as computer-readable instructions, data structures, program modules, or other data. Computer storage media includes RAM, ROM, EEPROM, solid state storage, flash memory or other memory technology, CD-ROM, digital versatile discs (DVDs) or other optical disk storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired information and which can be accessed by the computer  110 . Computer storage media does not include communication media. 
     Communication media typically embodies computer-readable instructions, data structures, program modules, or other data in a modulated data signal such as a carrier wave or other transport mechanism and includes any information delivery media. The term “modulated data signal” means a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media includes wired media such as a wired network or direct wired connection, and wireless media such as acoustic, RF, infrared and other wireless media. Combinations of any of the above should also be included within the scope of computer-readable media. 
     The system memory  130  includes computer storage media in the form of volatile and/or nonvolatile memory such as read only memory (ROM)  131  and random access memory (RAM)  132 . A basic input/output system  133  (BIOS), containing the basic routines that help to transfer information between elements within computer  110 , such as during start-up, is typically stored in ROM  131 . RAM  132  typically contains data and/or program modules that are immediately accessible to and/or presently being operated on by processing unit  120 . By way of example, and not limitation,  FIG. 1  illustrates operating system  134 , application programs  135 , other program modules  136 , and program data  137 . 
     The computer  110  may also include other removable/non-removable, volatile/nonvolatile computer storage media. By way of example only,  FIG. 1  illustrates a hard disk drive  141  that reads from or writes to non-removable, nonvolatile magnetic media, a magnetic disk drive  151  that reads from or writes to a removable, nonvolatile magnetic disk  152 , and an optical disc drive  155  that reads from or writes to a removable, nonvolatile optical disc  156  such as a CD ROM, DVD, or other optical media. Other removable/non-removable, volatile/nonvolatile computer storage media that can be used in the exemplary operating environment include magnetic tape cassettes, flash memory cards and other solid state storage devices, digital versatile discs, other optical discs, digital video tape, solid state RAM, solid state ROM, and the like. The hard disk drive  141  may be connected to the system bus  121  through the interface  140 , and magnetic disk drive  151  and optical disc drive  155  may be connected to the system bus  121  by an interface for removable nonvolatile memory such as the interface  150 . 
     The drives and their associated computer storage media, discussed above and illustrated in  FIG. 1 , provide storage of computer-readable instructions, data structures, program modules, and other data for the computer  110 . In  FIG. 1 , for example, hard disk drive  141  is illustrated as storing operating system  144 , application programs  145 , other program modules  146 , and program data  147 . Note that these components can either be the same as or different from operating system  134 , application programs  135 , other program modules  136 , and program data  137 . Operating system  144 , application programs  145 , other program modules  146 , and program data  147  are given different numbers herein to illustrate that, at a minimum, they are different copies. 
     A user may enter commands and information into the computer  110  through input devices such as a keyboard  162  and pointing device  161 , commonly referred to as a mouse, trackball, or touch pad. Other input devices (not shown) may include a microphone (e.g., for inputting voice or other audio), joystick, game pad, satellite dish, scanner, a touch-sensitive screen, a writing tablet, a camera (e.g., for inputting gestures or other visual input), or the like. These and other input devices are often connected to the processing unit  120  through a user input interface  160  that is coupled to the system bus, but may be connected by other interface and bus structures, such as a parallel port, game port or a universal serial bus (USB). 
     Through the use of one or more of the above-identified input devices a Natural User Interface (NUI) may be established. A NUI, may rely on speech recognition, touch and stylus recognition, gesture recognition both on screen and adjacent to the screen, air gestures, head and eye tracking, voice and speech, vision, touch, gestures, machine intelligence, and the like. Some exemplary NUI technology that may be employed to interact with a user include touch sensitive displays, voice and speech recognition, intention and goal understanding, motion gesture detection using depth cameras (such as stereoscopic camera systems, infrared camera systems, RGB camera systems, and combinations thereof), motion gesture detection using accelerometers/gyroscopes, facial recognition, 3D displays, head, eye, and gaze tracking, immersive augmented reality and virtual reality systems, as well as technologies for sensing brain activity using electric field sensing electrodes (EEG and related methods). 
     A monitor  191  or other type of display device is also connected to the system bus  121  via an interface, such as a video interface  190 . In addition to the monitor, computers may also include other peripheral output devices such as speakers  197  and printer  196 , which may be connected through an output peripheral interface  195 . 
     The computer  110  may operate in a networked environment using logical connections to one or more remote computers, such as a remote computer  180 . The remote computer  180  may be a personal computer, a server, a router, a network PC, a peer device or other common network node, and typically includes many or all of the elements described above relative to the computer  110 , although only a memory storage device  181  has been illustrated in  FIG. 1 . The logical connections depicted in  FIG. 1  include a local area network (LAN)  171  and a wide area network (WAN)  173 , but may also include phone networks, near field networks, and other networks. Such networking environments are commonplace in offices, enterprise-wide computer networks, intranets, and the Internet. 
     When used in a LAN networking environment, the computer  110  is connected to the LAN  171  through a network interface or adapter  170 . When used in a WAN networking environment, the computer  110  may include a modem  172  or other means for establishing communications over the WAN  173 , such as the Internet. The modem  172 , which may be internal or external, may be connected to the system bus  121  via the user input interface  160  or other appropriate mechanism. In a networked environment, program modules depicted relative to the computer  110 , or portions thereof, may be stored in the remote memory storage device. By way of example, and not limitation,  FIG. 1  illustrates remote application programs  185  as residing on memory device  181 . It will be appreciated that the network connections shown are exemplary and other means of establishing a communications link between the computers may be used. 
     Durable Program Execution 
     As mentioned previously, a program may have periods of time between events that involve processing. For example, a program that facilitates a workflow that involves humans may have days or weeks between events. For example, in an editing workflow, a writing project may be commissioned on one day and it may be several days or months before any activity on the writing project happens. After a draft is completed, it may be several days before editing commences. Even in programs that deal with strictly automated processes, the time between events may be long (e.g., relative to CPU time) even if the events are only minutes or seconds apart. 
       FIG. 2  is a block diagram that generally represents exemplary components for durable program execution in accordance with aspects of the subject matter described herein. The components illustrated in  FIG. 2  are exemplary and are not meant to be all-inclusive of components that may be needed or included. Furthermore, the number of components may differ in other embodiments without departing from the spirit or scope of aspects of the subject matter described herein. In some embodiments, the components described in conjunction with  FIG. 2  may be included in other components (shown or not shown) or placed in subcomponents without departing from the spirit or scope of aspects of the subject matter described herein. In some embodiments, the components and/or functions described in conjunction with  FIG. 2  may be distributed across multiple devices. 
     As used herein, the term component is to be read to include hardware such as all or a portion of a device, a collection of one or more software modules or portions thereof, some combination of one or more software modules or portions thereof and one or more devices or portions thereof, and the like. 
     For example, the components illustrated in  FIG. 2  may be implemented using one or more computing devices. Such devices may include, for example, personal computers, server computers, hand-held or laptop devices, multiprocessor systems, microcontroller-based systems, set-top boxes, programmable consumer electronics, network PCs, minicomputers, mainframe computers, cell phones, personal digital assistants (PDAs), gaming devices, printers, appliances including set-top, media center, or other appliances, automobile-embedded or attached computing devices, other mobile devices, distributed computing environments that include any of the above systems or devices, and the like. 
     An exemplary device that may be configured to implement one or more of the components of  FIG. 2  comprises the computer  110  of  FIG. 1 . 
     A component may also include or be represented by code. Code includes instructions that indicate actions a computer is to take. Code may also include information other than actions the computer is to take such as data, resources, variables, definitions, relationships, associations, and the like. 
     Code may be executed by a computer. When code is executed by a computer, this may be called a process. The term “process” and its variants as used herein may include one or more traditional processes, threads, components, libraries, objects that perform tasks, and the like. A process may be implemented in hardware, software, or a combination of hardware and software. In an embodiment, a process is any mechanism, however called, capable of or used in performing an action. A process may be distributed over multiple devices or a single device. Code may execute in user mode, kernel mode, some other mode, a combination of the above, or the like. 
     As illustrated, the environment  200  includes asynchronous components  205 - 207 , a code store  210 , a history store  211 , an event handler  215 , a controller  220 , a function invoker  230  and may include other components (not shown). A user interacting with the system is shown as user  225 , but in another embodiment, the user may be replaced with a process, system, or the like, that invokes the code involving asynchronous operations. 
     The code store  210  and the history store  211  include any storage media capable of storing data. A store may include computer storage elements such as volatile memory (e.g., RAM or other volatile memory described previously) and/or non-volatile memory (e.g., hard drives or other non-volatile memory described previously). A store may be located entirely on one device or may be distributed across multiple devices. A store may be external, internal, or include components that are both internal and external to a storage system hosting the store. A store may include components to ensure that the store is transactional and durable. 
     The term data is to be read broadly to include anything that may be represented by one or more computer storage elements. Logically, data may be represented as a series of l&#39;s and  0 &#39;s in volatile or non-volatile memory. In computers that have a non-binary storage medium, data may be represented according to the capabilities of the storage medium. Data may be organized into different types of data structures including simple data types such as numbers, letters, and the like, hierarchical, linked, or other related data types, data structures that include multiple other data structures or simple data types, and the like. Some examples of data include information, program code, program state, program data, other data, and the like. 
     The code store  210  stores code that is to be executed by the function invoker  230 . This code may be in virtually any language, scripting, non-scripting, or otherwise. 
     The function invoker  230  is structured (e.g., via code and/or hardware) to execute and re-execute code from the code store  210 . In executing code from the code store, an asynchronous operation may be encountered. An asynchronous operation is an operation that may complete independently of the main program flow. For example, a program may initiate an asynchronous operation by requesting a Web page. The request for a Web page may be serviced by another machine that may take a varying amount of time to service the request. 
     In one sense, a response is data that indicates that something has been done to satisfy an asynchronous operation. A response may include more or less information. In the Web page example above, a response may include the data of a Web page. Responses to other asynchronous operations, however, may include relatively little information. For example, a response may indicate that an asynchronous component was contacted and instructed to execute. A response may include a return code or other data provided by the asynchronous component. A response may include an error code that indicates that an error occurred in trying to satisfy an asynchronous operation. 
     When the function invoker  230  encounters an asynchronous operation in the code, first, the function invoker  230  may search the history store  211  for an indication of a completion of the asynchronous request. If a completion record is found, the function invoker  230  may provide completion data to the code. Completion data may indicate that the asynchronous operation has been executed, completed, and/or failed and may include response data, if any. If a completion record is not found, the function invoker  230  may send a command to the controller  220 . The command may provide information about the operation and allow the controller  220  to process the asynchronous operation. 
     When the controller  220  receives a command from the function invoker  230 , the controller  220  may process the command. In processing the command, the controller  220  may start an asynchronous operation and update the history  211  to mark the asynchronous operation as started. The controller  220  may perform these actions in the same transaction. To start an asynchronous operation controller  220  may schedule a task to cause a request to be made to an asynchronous component suitable for the asynchronous operation. 
     The asynchronous components  205 - 207  may include computer components or other entities that service asynchronous operations. A Web server has been previously described as one such component, but those skilled in the art will recognize many other components that may service asynchronous requests. For example, in one embodiment, the asynchronous components  205 - 207  may include only automated components that are built using hardware and/or software. In another embodiment, the asynchronous components  205 - 207  may include both automated components and humans or other entities. In some embodiments, the asynchronous components  205 - 207  may include computer components that obtain input from humans and/or other entities. For example, the servicing components may exist as code that executes on computers that reminds people involved in a workflow that certain tasks are to be completed and provided so that the workflow may proceed to completion. 
     If the history  211  does not include a completion record for the asynchronous operation, the function invoker  230  may continue to run the code, if any, that appears after the asynchronous operation unless the code is blocked on the asynchronous operation completing in which case the function invoker  230  may halt execution of the program and remove the program from memory without storing any additional state about the program. 
     When a response is received, the event handler  215  may update the history store  211  to indicate that an asynchronous component has responded to the asynchronous operation. The event handler  215  may also notify the function invoker  230  that a response has been received regarding the code previously executed. After receiving the notification, the function invoker  230  may re-execute the code from the start. When the asynchronous operation is executed again, the function invoker  230  may find the completion data in the history store  211 . The function invoker  230  may supply the completion data to the code which may then complete the asynchronous operation. 
     The process described above may be repeated as many times as the code includes asynchronous operations. Eventually, the code may execute to completion and have all asynchronous requests completed during the last execution. 
     The above process allows the code and state to be removed from memory between executions and allows a software developer to write the code without worrying about saving state after handling each event. 
     Another example of how this works with some exemplary code is provided below. The exemplary code may include: 
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                 function(baseAddress) 
               
               
                   
                 { 
               
            
           
           
               
               
            
               
                   
                 var getUserUrl = baseAddress + ‘GetUser’; 
               
               
                   
                 var sendGrettingUrl = baseAddress + ‘SendGretting’; 
               
               
                   
                 return context.httpSendWithPromise( 
               
            
           
           
               
               
            
               
                   
                 { 
               
            
           
           
               
               
            
               
                   
                 Method: ‘Get’, 
               
               
                   
                 url:getUserUrl 
               
            
           
           
               
               
            
               
                   
                 }).then(function(getUserResponse){ 
               
               
                   
                 var user = getUserResponse.responseText; 
               
               
                   
                 return context.httpSendWithPromise( 
               
            
           
           
               
               
            
               
                   
                 { 
               
            
           
           
               
               
            
               
                   
                 Method: ‘Post’, 
               
               
                   
                 url:sendGreetingUrl, 
               
               
                   
                 body: user 
               
            
           
           
               
               
            
               
                   
                 }); 
               
            
           
           
               
               
            
               
                   
                 }); 
               
            
           
           
               
               
            
               
                   
                 } 
               
               
                   
                   
               
            
           
         
       
     
     The first time this code is invoked by the function invoker  230 , an entry may be made in the history store  211  that identifies that the program has started execution. When the instruction “return context.httpSendWithPromise” is encountered, this may cause the function invoker  230  to be called. In response, the function invoker  230  may search the history store  211  to see if a completion data corresponding to the asynchronous operation is recorded therein. 
     If the history store  211  does not include the completion data, the function invoker  230  may provide a command to the controller  220 . The command may include data regarding the asynchronous operation (e.g., Get and the getUserUrl). The controller  220  may then schedule a task to cause a request to be made to one of the asynchronous components  205 - 207  for the asynchronous operation. An entry may be made in the history store  211  that indicates that the task has been started. 
     When a response from the asynchronous component is received, the event handler  215  may store the completion data corresponding to the response in the history  211  and may update the history to indicate that the task has completed. The event handler  215  may also notify the function invoker  230  that a response has been received so that the function invoker  230  may re-executed the code. 
     When the code is re-executed, it may again encounter the instruction “return context.httpSendWithPromise”. When the instruction is encountered, this may again cause the function invoker  230  to be called. In response, the function invoker  230  may again search the history store  211  for a response. This time, the function invoker  230  finds the completion data previously stored by the event handler  215 . The function invoker  230  may then provide the completion data to the executing code which may then complete the asynchronous operation. The instructions of the “then(function(getUserResponse)” function may then be executed. 
     In the course of executing these instructions, the instruction “return context.httpSendWithPromise” may be executed. When this instruction is executed, the executing code may pass information to the function invoker  230 . The function invoker  230  may again search the history store  211  for completion data corresponding to the asynchronous operation. In finding that the completion data for the asynchronous operation is not available in the history store  211 , the function invoker  230  may send a command to the controller  220 . In response, the controller  220  may schedule a task to cause an asynchronous request to be made to an appropriate asynchronous component. 
     When the function invoker  230  determines that completion data for an asynchronous operation is not currently available is the history store  211 , the function invoker  230  may execute the code after the asynchronous operation (if the asynchronous operation is non-blocking) or halt the execution of the program and remove the program from memory without storing any additional state about the program. As indicated previously, the function invoker  230  may halt execution of a program if either of the two conditions below exist: 
     1. The end of the code has been reached; or 
     2. There is no response to an asynchronous operation AND the asynchronous operation is blocking. 
     In one example, the function invoker  230  may determine that a response to the asynchronous operation is currently not available if there are no more completion events in the history. 
     When a response to the second asynchronous request is received, the event handler  215  may update the history store  211  with completion data that includes the response received and may also notify the function invoker  230  that a response has been received regarding the code previously executed. In response, the function invoker  230  may again re-execute the code from the start of the code. As each asynchronous operation is executed from the executing code, the function invoker  230  may search the history  211 , find the completion data (if it is available), and provide the completion data to the executing code which may then continue executing. 
     In this case, the code may execute to completion as completion data for both asynchronous requests are found in the history store  211 . Upon completion, an entry may be logged that indicates that the code has completed. The entry may include the results returned by the executing code, if any. 
     Below are some exemplary entries that may be stored in the history store  211  corresponding to the actions described above: 
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                 StartExecution: (Input: [‘http://foo’, ...]) 
               
               
                   
                 HttpTaskStarted: (EventId: 1, Request: {...}} 
               
               
                   
                 HttpTaskCompleted: {StartedEventId: 1, Response: {...}} 
               
               
                   
                 HttpTaskStarted: (EventId: 2, Request: {...}} 
               
               
                   
                 HttpTaskCompleted: {StartedEventId: 2, Response: {...}} 
               
               
                   
                   
               
            
           
         
       
     
     Below are some other exemplary entries that may, in one embodiment, also be stored. These entries may be stored in the history store  211 , a task store (not shown), or some other store. 
     
       
         
           
               
             
               
                   
               
             
            
               
                 ScheduleHttpTask: {Id: 1, Request: {Url: ‘getUser’}} 
               
               
                 ScheduleHttpTask: {Id: 2, Request: {Url: ‘sendGreeting’, Body: {...}}} 
               
               
                 CompleteCode: {Id: 3, Result: ...} 
               
               
                   
               
            
           
         
       
     
     In another embodiment, these other exemplary entries are not persisted in a store. 
       FIGS. 3-6  are flow diagrams that generally represent exemplary actions that may occur in accordance with aspects of the subject matter described herein. For simplicity of explanation, the methodology described in conjunction with  FIGS. 3-6  is depicted and described as a series of acts. It is to be understood and appreciated that aspects of the subject matter described herein are not limited by the acts illustrated and/or by the order of acts. In one embodiment, the acts occur in an order as described below. In other embodiments, however, the acts may occur in parallel, in another order, and/or with other acts not presented and described herein. Furthermore, not all illustrated acts may be required to implement the methodology in accordance with aspects of the subject matter described herein. In addition, those skilled in the art will understand and appreciate that the methodology could alternatively be represented as a series of interrelated states via a state diagram or as events. 
       FIGS. 3-4  are flow diagrams that generally represent exemplary actions that may occur to obtain asynchronous operations from a program and responses thereto in accordance with aspects of the subject matter described herein. At block  300 , the actions begin. 
     At block  305 , program execution is commenced. For example, referring to  FIG. 2 , a program that is stored on the code store  210  (e.g., the program described above) is launched by the function invoker  230 . 
     At block  310 , an asynchronous operation is encountered in the program. For example, the following code is executed: 
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                 return context.httpSendWithPromise( 
               
               
                   
                 { 
               
            
           
           
               
               
            
               
                   
                 Method: ‘Get’, 
               
               
                   
                 url:getUserUrl 
               
            
           
           
               
               
            
               
                   
                 } 
               
               
                   
                   
               
            
           
         
       
     
     At block  315 , a history is checked to determine whether a response has been received to the asynchronous operation. For example, referring to  FIG. 2 , the function invoker  230  checks the history store  211  to determine whether a response has been received for the request. Checking the history may include searching the history of an identifier associated with the response. When the identifier is located, the data associated with the identifier may be retrieved from the history and synchronously provided to the executing program even though the data was received asynchronously to the first time the program encountered the asynchronous operation. 
     At block  320 , if a response has not been received, the actions continue at block  322 ; otherwise, the actions continue at block  325 . 
     At block  322 , a check is made as to whether a task is already scheduled for the encountered operation. If so, the actions continue at block  330 ; otherwise, the actions continue at block  323 . For example, in re-executing a program, an asynchronous operation may be encountered for which a task was scheduled but for which no response has yet been received. 
     At block  323 , a task is scheduled to make an asynchronous request. For example, referring to  FIG. 2 , the controller  220  may schedule a task to call one of the asynchronous components  205 - 207  in response to an asynchronous operation encountered within the program executed by the function invoker  230 . 
     At block  325 , completion data is obtained from the history and provided to the program. For example, referring to  FIG. 2 , the function invoker  230  obtains the completion data from the history store  211  and provides the completion data to the executing program. 
     At block  330 , if another asynchronous request is encountered while executing the program, then the actions continue at block  315 ; otherwise, the actions continue at block  335 . 
     At block  335 , a determination is made as to whether there are any pending asynchronous task(s). If so, the actions continue at block  410  of  FIG. 4 ; otherwise, the actions continue at block  340  of  FIG. 3 . 
     At block  340 , the result (e.g., the return code or data) from the program is returned. For example, referring to  FIG. 3 , the function invoker  230  returns the results of executing the code to the user  225  via a display. 
     At block  340 , other actions, if any, may be performed. 
     Turning to  FIG. 4 , in one embodiment, at block  410 , the program may be removed from memory if conditions described previously are met. For example, referring to  FIG. 2 , the function invoker  230  may remove that executing program from memory. Removing may include marking the memory as free, instructing another component to kill the program, or any other activity that indicates that the program may be stopped and the memory re-used for other purposes. In one embodiment, the program may be marked for deletion but not removed unless it is determined that more resources are needed. 
     At block  415 , one or more tasks from the task list are executed. For example, referring to  FIG. 2 , the controller  220  (or another component) executes a task that was previously placed in the history store  211  and sends a request to one of the asynchronous components  205 - 207 . 
     At block  420 , waiting for the response is performed. Waiting may involve doing nothing or engaging in other actions related or non-related to the response. For example, waiting may involve spawning a thread to wait for the response while also executing other threads that do not depend on the response. For example, referring to  FIG. 2 , the function invoker  230  may execute other code and the other components may engage in other activities while waiting for a response. 
     At block  425 , when the response is received, it is placed in a history. For example, referring to  FIG. 2 , the event handler  215  may place completion data corresponding to the response received from the appropriate asynchronous component into the history store  211 . Placing the data into the history may involve storing the completion data corresponding to the response together with an identifier that associates the completion data with the asynchronous operation and/or the program executing the asynchronous operation. 
     At block  430 , a notification is provided that a response has been received. For example, referring to  FIG. 2 , the event handler  215  may notify the function invoker  230  that a response has been received and may indicate the program for which the response was received. 
     After block  430 , the actions continue at block  305  of  FIG. 3 . 
       FIG. 5  is a flow diagram that generally represents exemplary actions that may occur in providing code to an execution environment in accordance with aspects of the subject matter described herein. At block  505 , the actions begin. 
     At block  510 , code of a program is provided to an execution environment. For example, referring to  FIG. 2 , the user  225 , a computer process, or the like may provide an indication to the function invoker  230  of the existence of program that is to be executed. The indication may indicate the name, location, or other information that identifies the program. For example, the indication may identify code stored in the code store  210 . 
     At block  515 , after the code of the program has been executed and re-executed until all asynchronous operations of the program have been satisfied by the history, results regarding the execution of the program are received. For example, referring to  FIG. 2 , the user  225 , a computer process, or the like may receive an indication of the results of executing the program. This result may be received after the program is able to successfully execute from beginning to end having responses to all of its asynchronous requests. 
     At block  520 , other actions, if any, may be performed. 
       FIG. 6  is a flow diagram that generally represents exemplary actions that may occur for a function invoker in accordance with aspects of the subject matter described herein. At block  600 , the actions begin. 
     At block  605 , the function invoker is launched. For example, referring to  FIG. 2 , the function invoker  230  may be launched by the event handler  215  after a new event for a program is received by the event handler  215 . As another example, the function invoker  230  may be launched by the user  225  or a process (not shown). 
     At block  610 , after being launched, the function invoker may obtain an event in the history. For example, referring to  FIG. 2 , the function invoker  230  may obtain the first event in the history store  211 . 
     At block  615 , the function invoker may process the event. For example, if the event is a completion event that corresponds to an asynchronous operation indicated by a program, the function invoker may provide completion data of the completion event to the program. 
     At block  620 , the function invoker determines if there is another event in the history, the actions continue at block  610 ; otherwise, the actions continue at block  625 . For example, referring to  FIG. 2 , if the history store  211  includes several events, the function invoker  230  may determine if there is another event that the function invoker  230  has not processed. If so, the actions continue at block  610 ; otherwise, the actions continue at block  625 . 
     At block  625 , other actions if any, are performed. 
     As can be seen from the foregoing detailed description, aspects have been described related to durable program execution. While aspects of the subject matter described herein are susceptible to various modifications and alternative constructions, certain illustrated embodiments thereof are shown in the drawings and have been described above in detail. It should be understood, however, that there is no intention to limit aspects of the claimed subject matter to the specific forms disclosed, but on the contrary, the intention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of various aspects of the subject matter described herein.