Project: MongoDB (App Definition and Development / Database)
File: logging.md

The new log system adds capability to produce structured logs in the [Relaxed Extended JSON 2.0.0][relaxedjson2] format. The new API requires names to be given to variables, forming field names for the variables in structured JSON logs. Named variables are called attributes in the log system. Log lines are composed primarily of a message (`msg`) and attributes (`attr` fields). As you write log messages, keep the following in mind: A big thing that makes JSON and BSON useful as data formats is the ability to provide rich field names. What makes logv2 machine readable is that we write an intact Extended BSON format. But, what makes these lines human readable is that the `msg` provides a simple, clear context for interpreting well-formed field names and values in the `attr` subdocument. For maximum readability, a log message additionally has the least amount of repetition possible, and shares attribute names with other related log lines. The `msg` field predicates a reader's interpretation of the log line. It should be crafted with care and attention. Concisely describe what the log line is reporting, providing enough context necessary for interpreting attribute field names and values Capitalize the first letter, as in a sentence Avoid unnecessary punctuation, but punctuate between sentences if using multiple sentences Do not conclude with punctuation You may occasionally encounter `msg` strings containing fmt-style `{expr}` braces. These are legacy artifacts and should be rephrased according to these guidelines. The `attr` subdocument includes important metrics/statistics about the logged event for the purposes of debugging or performance analysis. These variables should be named very well, as though intended for a very human-readable portion of the codebase (like config variable declaration, abstract class definitions, etc.) For `attr` field names, do the following: The bar for understanding should be: Someone with reasonable understanding of mongod behavior should understand immediately what is being logged Someone with reasonable troubleshooting skill should be able to extract doc- or code-searchable phrases to learn about what is being logged Exception: Do not add a unit suffix when logging a Duration type. The system automatically adds this unit. To describe the execution time of an operation using our preferred method: Specify an `attr` name of duration and provide a value using the Milliseconds Duration type. The log system will automatically append "Millis" to the attribute name. Alternatively, specify an `attr` name of durationMillis and provide the number of milliseconds as an integer type. Importantly: downstream analysis tools will rely on this convention, as a replacement for the "[0-9]+ms$" format of prior logs. When logging the below information, do so with these specific terms: namespace - when logging a value of the form "\<db name\>.\<collection name\>". Do not use "collection" or abbreviate to "ns" db - instead of "database" error - when an error occurs, instead of "status". Use this for objects of type Status and DBException reason - to provide rationale for an event/action when "error" isn't appropriate Example 1: LOGV2(1041, "Transition to PRIMARY complete"); JSON Output: { ... , "id": 1041, "msg": "Transition to PRIMARY complete", "attr": {} } Example 2: LOGV2(1042, "Slow query", "duration"_attr = getDurationMillis()); JSON Output: {

"id": 1042, "msg": "Slow query", "attr": { "durationMillis": 1000 } } For adding STL containers as dynamic attributes, see For sharing a string between a log line and a status see [this section of InitialSyncer::\lastOplogEntryFetcherCallbackForStopTimestamp][ lastOplogEntryFetcherCallbackForStopTimestamp] The log system is made available with the following header: The macro `MONGOLOGV2DEFAULT_COMPONENT` is expanded by all logging macros. This configuration macro must expand at their point of use to a `LogComponent` expression, which is implicitly attached to the emitted message. It is conventionally defined near the top of a `.cpp` file after headers are included, and before any logging macros are invoked. Example: Logging is performed using function style macros: LOGV2(ID, message-string, "name0"_attr = var0, ..., "nameN"_attr = varN); The ID is a signed 32bit integer in the same number space as the error code numbers. It is used to uniquely identify a log statement. If changing existing code, using a new ID is strongly advised to avoid any parsing ambiguity. When selecting ID during work on JIRA ticket `SERVER-ABCDE` you can use the JIRA ticket number to avoid ID collisions with other engineers by taking ID from the range `ABCDE00` - `ABCDE99`. Replacement fields are placed in the format string with curly braces `{}`. Everything not surrounded with curly braces is part of the message text. Curly brace characters can be output by escaping them using double braces: `{{` or `}}`. Attributes are created with the `_attr` user-defined literal. The intermediate object that gets instantiated provides the assignment operator `=` for assigning a value to the attribute. Attributes are associated with replacement fields in the format string by name or index, using names is strongly recommended. When using unnamed replacement fields, attributes map to replacement fields in the order they appear in the format string. It is allowed to have more attributes than replacement fields in a log statement. However, having fewer attributes than replacement fields is not allowed. The message string must be a compile time constant. This is to avoid dynamic attribute names in the log output and to be able to add compile time verification of log statements in the future. If the string needs to be shared with anything else (like constructing a Status object) you can use this pattern: static constexpr char str[] = "the string"; No attributes. LOGV2(1000, "Logging event, no replacement fields is OK"); Some attributes. LOGV2(1002, "Replication state change", "from"_attr = getOldState(), "to"_attr = getNewState()); To override the default component, a separate logging API can be used that takes a `LogOptions` structure: LOGV2_OPTIONS(options, message-string, attr0, ...); `LogOptions` can be constructed with a `LogComponent` to avoid verbosity in the log statement. LOGV2_OPTIONS(1003, {LogComponent::kCommand}, "To the command component"); `LOGV2` is the logging macro for the default informational (0) severity. To log to different severities there are separate logging macros to be used, they all take paramaters like `LOGV2`: `LOGV2_WARNING` `LOGV2_ERROR` `LOGV2_FATAL` `LOGV2FATALNOTRACE` `LOGV2FATALCONTINUE` There is also variations that take `LogOptions` if needed: `LOGV2WARNINGOPTIONS` `LOGV2ERROROPTIONS` `LOGV2FATALOPTIONS` Fatal level log statements using `LOGV2_FATAL` perform `fassert` after logging, using the provided ID as assert id. `LOGV2FATALNOTRACE` perform `fassertNoTrace` and `LOGV2FATALCONTINUE` does not `fassert` allowing for continued execution. `LOGV2FATALCONTINUE` is meant to be used when a fatal error has occured but a different way of halting execution is desired such as `std::terminate` or `fassertFailedWithStatus`. `LOGV2FATALOPTIONS` performs `fassert` by default like `LOGV2_FATAL` but this can be changed by setting the `FatalMode` on the `LogOptions`. Debug-level logging is slightly different where an additional parameter (as integer) required to indicate the desired debug level: LOGV2_DEBUG(ID, debug-level, format-string, attr0, ...); LOGV2_DEBUG(ID, debug-level, format-string, message-string, attr0,

LOGV2DEBUGOPTIONS( ID, debug-level, options, format-string, attr0, ...); LOGV2DEBUGOPTIONS( ID, debug-level, options, format-string, message-string, attr0, ...); Status status = ...; int remainingAttempts = ...; LOGV2_ERROR(1004, "Initial sync failed.", "reason"_attr = status, "remainingAttempts"_attr = remainingAttempts); Log tags are replacing the Tee from the old log system as the way to indicate that the log should also be written to a `RamLog` (accessible with the `getLog` command). Tags are added to a log statement with the options API similarly to how non-default components are specified by constructing a `LogOptions`. Multiple tags can be attached to a log statement using the bitwise or operator `|`. LOGV2WARNINGOPTIONS( 1005, {LogTag::kStartupWarnings}, "XFS filesystem is recommended with WiredTiger"); Sometimes there is a need to add attributes depending on runtime conditionals. To support this there is the `DynamicAttributes` class that has an `add` method to add named attributes one by one. This class is meant to be used when you have this specific requirement and is not the general logging API. When finished, it is logged using the regular logging API but the `DynamicAttributes` instance is passed as the first attribute parameter. Mixing `_attr` literals with the `DynamicAttributes` is not supported. When using the `DynamicAttributes` you need to be careful about parameter lifetimes. The `DynamicAttributes` binds attributes by reference and the reference must be valid when passing the `DynamicAttributes` to the log statement. DynamicAttributes attrs; attrs.add("str", "StringData value"_sd); if (condition) { // getExtraInfo() returns a reference that is valid until the LOGV2 // call below. Be careful of functions returning by value. attrs.add("extra", getExtraInfo()); } LOGV2(1030, "Dynamic attributes", attrs); Many basic types have built in support: Boolean Integral types Single `char` is logged as integer Enums Logged as their underlying integral type Floating point types `long double` is prohibited String types `std::string` `StringData` `const char*` Duration types Special formatting, see below BSON types `BSONObj` `BSONArray` `BSONElement` BSON appendable types `BSONObjBuilder::append` overload available `boost::optional<T>` of any loggable type `T` To make a user-defined type loggable it needs a serialization member function that the log system can bind to. The system binds and uses serialization functions by looking for functions in the following priority order: Structured serialization functions `void x.serialize(BSONObjBuilder*) const` (member) `BSONObj x.toBSON() const` (member) `BSONArray x.toBSONArray() const` (member) `toBSON(x)` (non-member) Stringification functions `toStringForLogging(x)` (non-member) `x.serialize(&fmtMemoryBuffer)` (member) `x.toString() ` (member) `toString(x)` (non-member) Enums cannot have member functions, but they will still try to bind to the `toStringForLogging(e)` or `toString(e)` non-members. If neither is available, the enum value will be logged as its underlying integral type. In order to offer structured serialization and output, a type would need to supply a structured serialization function. Otherwise, if only stringification is provided, the output will be an escaped string. The `toStringForLogging` non-member is an ADL customization hook used to override `toString` for very rare cases where `toString` is inappropriate for logging perhaps because it's needed for other non-logging formatting. Usually a `toString` (member or nonmember) is a sufficient customization point and should be preferred as a canonical stringification of the object. NOTE: No `operator<<` overload is used even if available class UserDefinedType { public: void serialize(BSONObjBuilder* builder) const { builder->append("str"sd, str); builder->append("int"sd, int); } private: std::string _str; int32t int; }; STL containers and data structures that have STL like interfaces are loggable as long as they contain loggable elements (built-in, user-defined or other

Sequential containers like `std::vector`, `std::deque` and `std::list` are loggable and the elements get formatted as JSON array in structured output. Associative containers such as `std::map` and `stdx::unordered_map` loggable with the requirement that they key is of a string type. The structured format is a JSON object where the field names are the key. Ranges is loggable via helpers to indicate what type of range it is `seqLog(begin, end)` `mapLog(begin, end)` seqLog indicates that it is a sequential range where the iterators point to loggable value directly. mapLog indicates that it is a range coming from an associative container where the iterators point to a key-value pair. Logging a sequence: std::array<int, 20> arrayOfInts = ...; LOGV2(1010, "Log container directly", "values"_attr = arrayOfInts); LOGV2(1011, "Log iterator range", "values"_attr = seqLog(arrayOfInts.begin(), arrayOfInts.end()); LOGV2(1012, "Log first five elements", "values"_attr = seqLog(arrayOfInts.data(), arrayOfInts.data() + 5); Logging a map-like container: StringMap<BSONObj> bsonMap = ...; LOGV2(1013, "Log map directly", "values"_attr = bsonMap); LOGV2(1014, "Log map iterator range", "values"_attr = mapLog(bsonMap.begin(), bsonMap.end()); Logging of containers uses `BSONObj` as an internal representation and `uint64_t` is not a supported type with `BSONObjBuilder::append()`. As a user you can use `boost::transformiterator` to cast the `uint64t` to a supported type. std::vector<uint64_t> vec = ...; // If we know casting to signed is safe auto asSigned = { return staticcast<int64t>(i); }; LOGV2(2000, "As signed array", "values"_attr = seqLog( boost::maketransformiterator(vec.begin(), asSigned), boost::maketransformiterator(vec.end(), asSigned))); // Otherwise we can log as any of these types instead of using asSigned auto asDecimal128 = { return Decimal128(i); }; auto asString = { return std::to_string(i); }; Duration types have special formatting to match existing practices in the server code base. Their resulting format depends on the context they are logged. When durations are formatted as JSON or BSON a unit suffix is added to the attribute name when building the field name. The value will be count of the duration as a number. When logging containers with durations there is no attribute per duration instance that can have the suffix added. In this case durations are instead formatted as a BSON object. "duration" attribute C++ expression: "duration"_attr = Milliseconds(10) JSON format: "durationMillis": 10 Container of Duration objects C++ expression: "samples"_attr = std::vector<Nanoseconds>{Nanoseconds(200), Nanoseconds(400)} JSON format: "samples": [{"durationNanos": 200}, {"durationNanos": 400}] The style guide contains recommendations for attribute naming in certain cases. To make abstraction of attribute naming possible a `logAttrs` function can be implemented as a friend function in a class with the following signature: class AnyUserType { public: friend auto logAttrs(const AnyUserType& instance) { return "name"_attr=instance; } BSONObj toBSON() const; // Type needs to be loggable }; const AnyUserType& t = ...; LOGV2(2001, "Log of user type", logAttr(t)); In some cases a loggable type might be composed as a hierarchy in the C++ type system which would lead to a very verbose structured log output as every level in the hierarcy needs a name when outputted as JSON. The attribute naming abstraction system can also be used to collapse such hierarchies. Instead of making a type loggable it can instead return one or more attributes from its members by using `multipleAttrs` in `logAttrs` functions. `multipleAttrs(...)` accepts attributes or instances of types with `logAttrs` functions implemented. class NotALoggableType { std::string name; BSONObj data; friend auto logAttrs(const NotALoggableType& instance) { return logv2::multipleAttrs("name"_attr=instance.name, "data"_attr=instance.data); } }; NotALoggableType t = ...; // These two log statements are equivalent: LOGV2(2002, "Statement", logAttrs(t)); LOGV2(2002, "Statement", "name"attr=t.name,

To avoid lifetime issues (log attributes bind their values by reference) it is recommended to not create attributes when using `multipleAttrs` unless attributes are created for members directly. If `logAttrs` or `""_attr=` is used inside a `logAttrs` function on the return of a function returning by value it will result in a dangling reference. The following example illustrates the problem: class SomeSubType { public: BSONObj toBSON() const {...}; friend auto logAttrs(const SomeSubType& sub) { return "subAttr"_attr=sub; } }; class SomeType { public: const std::string& name() const { return name_; } SomeSubType sub() const { return sub_; } // Returning by value! friend auto logAttrs(const SomeType& type) { // logAttrs(type.sub()) below will contain a dangling reference! return logv2::multipleAttrs("name"_attr=type.name(), logAttrs(type.sub())); } private: SomeSubType sub_; std::string name_; }; The better implementation would be to let the log system control the lifetime by passing the instance to `multipleAttrs` without creating the attribute. The log system will detect that it is not an attribute and will attempt to create attributes by calling `logAttrs`: friend auto logAttrs(const SomeType& type) { return logv2::multipleAttrs("name"_attr=type.name(), type.sub()); } Code that emits a high severity log statement may also need to emit a `uassert` after the log. There is the `UserAssertAfterLog` logging option that allows you to re-use the log statement to do the formatting required for the `uassert`. The assertion id can be either the logging ID by passing `UserAssertAfterLog` with no arguments or the assertion id can set by constructing `UserAssertAfterLog` with an `ErrorCodes::Error`. The assertion reason string will be a plain text formatted log (replacement fields filled in format-string). If replacement fields are not provided in the message string, attribute values will be missing from the assertion message. LOGV2ERROROPTIONS(1050000, {UserAssertAfterLog()}, "Assertion after log"); Would emit a `uassert` after performing the log that is equivalent to: uasserted(1050000, "Assertion after log"); Using a named error code: LOGV2ERROROPTIONS( 1050, {UserAssertAfterLog(ErrorCodes::DataCorruptionDetected)}, "Data corruption detected", "recordId"_attr=RecordId(123456)); Would emit a `uassert` after performing the log that is equivalent to: uasserted(ErrorCodes::DataCorruptionDetected, "Data corruption detected for RecordId(123456)"); To make it easier to use the log system for tracing in local development, there is a special API that does not use IDs or attribute names: logd(format-string, value0, ..., valueN); It formats the string using libfmt similarly to what `fmt::format(format-string, value0, ..., valueN)` would produce but using the regular log system type support on how types are made loggable. The formatted string is logged as the `msg` field in the JSON output, with no `attr` subobject. When using `logd` the log will emitted with standard severity and the default component. A difference from regular logging, `logd` is allowed to be used in header files by including `logv2/log_debug.h`. Unstructured logging is not allowed to be used in code committed to master, there is a lint check to validate this. It is however allowed to be used in Evergreen patch builds. UserDefinedType t; // Defined in previous example logd("this is a debug log, value 1: {} and value 2: {}", 1, t); Produces structured logs of the format. Below is an example of a log statement in C++ and a pretty-printed JSON output: C++ statement: BSONObjBuilder builder; builder.append("first"_sd, 1); builder.append("second"_sd, "str"); std::vector<int> vec = {1, 2, 3}; LOGV2_ERROR(1020, "Example", "bson"_attr = builder.obj(), "vector"_attr = vec, "optional"_attr = boost::none); Output: { "t": { "$date": "2020-01-06T19:10:54.246Z" }, "s": "E", "c": "NETWORK", "ctx": "conn1", "id": 23453, "msg": "Example", "attr": { "bson": { "first": 1, "second": "str" }, "vector": [1, 2, 3], "optional": null } }