A method comprising: receiving, from a client device, a digital component request comprising a set of data; in response to receiving the digital component request: identifying a multi-stage workflow for selecting a digital component from candidate digital components of multiple content platforms based on the set of data, wherein the multi-stage workflow comprises one or more customizable stages; executing each stage of the multi-stage workflow in a sequence defined by the multi-stage workflow, the executing comprising: identifying a given customizable stage in the multi-stage workflow; for the given customizable stage: identifying, for each of the multiple content platforms, a customization specific to the given stage provided by the content platform, wherein each customization comprises computer-executable code that generates an output for use in selecting the digital component; initiating an isolated execution environment for each customization; executing, within each isolated execution environment, the computer-executable code of the customization for which the isolated execution environment was initiated; and obtaining the output generated by the computer-executable code of each isolated execution environment; and executing a final stage to select a digital component based on the obtained outputs; and sending the selected digital component to the client device. receiving, from a client device, a digital component request comprising a set of data; in response to receiving the digital component request: identifying a multi-stage workflow for selecting a digital component from candidate digital components of multiple content platforms based on the set of data, wherein the multi-stage workflow comprises one or more customizable stages; executing each stage of the multi-stage workflow in a sequence defined by the multi-stage workflow, the executing comprising: identifying a given customizable stage in the multi-stage workflow; for the given customizable stage: identifying, for each of the multiple content platforms, a customization specific to the given stage provided by the content platform, wherein each customization comprises computer-executable code that generates an output for use in selecting the digital component; initiating an isolated execution environment for each customization; executing, within each isolated execution environment, the computer-executable code of the customization for which the isolated execution environment was initiated; and obtaining the output generated by the computer-executable code of each isolated execution environment; and executing a final stage to select a digital component based on the obtained outputs; and sending the selected digital component to the client device. identifying a multi-stage workflow for selecting a digital component from candidate digital components of multiple content platforms based on the set of data, wherein the multi-stage workflow comprises one or more customizable stages; executing each stage of the multi-stage workflow in a sequence defined by the multi-stage workflow, the executing comprising: identifying a given customizable stage in the multi-stage workflow; for the given customizable stage: identifying, for each of the multiple content platforms, a customization specific to the given stage provided by the content platform, wherein each customization comprises computer-executable code that generates an output for use in selecting the digital component; initiating an isolated execution environment for each customization; executing, within each isolated execution environment, the computer-executable code of the customization for which the isolated execution environment was initiated; and obtaining the output generated by the computer-executable code of each isolated execution environment; and executing a final stage to select a digital component based on the obtained outputs; and identifying a given customizable stage in the multi-stage workflow; for the given customizable stage: identifying, for each of the multiple content platforms, a customization specific to the given stage provided by the content platform, wherein each customization comprises computer-executable code that generates an output for use in selecting the digital component; initiating an isolated execution environment for each customization; executing, within each isolated execution environment, the computer-executable code of the customization for which the isolated execution environment was initiated; and obtaining the output generated by the computer-executable code of each isolated execution environment; and identifying, for each of the multiple content platforms, a customization specific to the given stage provided by the content platform, wherein each customization comprises computer-executable code that generates an output for use in selecting the digital component; initiating an isolated execution environment for each customization; executing, within each isolated execution environment, the computer-executable code of the customization for which the isolated execution environment was initiated; and obtaining the output generated by the computer-executable code of each isolated execution environment; and executing a final stage to select a digital component based on the obtained outputs; and sending the selected digital component to the client device.