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https://github.com/distribution/distribution
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89 lines
3.8 KiB
Go
89 lines
3.8 KiB
Go
// Package context provides several utilities for working with
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// Go's context in http requests. Primarily, the focus is on logging relevant
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// request information but this package is not limited to that purpose.
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//
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// The easiest way to get started is to get the background context:
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//
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// ctx := context.Background()
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//
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// The returned context should be passed around your application and be the
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// root of all other context instances. If the application has a version, this
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// line should be called before anything else:
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//
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// ctx := context.WithVersion(context.Background(), version)
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//
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// The above will store the version in the context and will be available to
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// the logger.
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//
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// Logging
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//
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// The most useful aspect of this package is GetLogger. This function takes
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// any context.Context interface and returns the current logger from the
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// context. Canonical usage looks like this:
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//
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// GetLogger(ctx).Infof("something interesting happened")
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//
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// GetLogger also takes optional key arguments. The keys will be looked up in
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// the context and reported with the logger. The following example would
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// return a logger that prints the version with each log message:
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//
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// ctx := context.Context(context.Background(), "version", version)
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// GetLogger(ctx, "version").Infof("this log message has a version field")
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//
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// The above would print out a log message like this:
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//
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// INFO[0000] this log message has a version field version=v2.0.0-alpha.2.m
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//
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// When used with WithLogger, we gain the ability to decorate the context with
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// loggers that have information from disparate parts of the call stack.
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// Following from the version example, we can build a new context with the
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// configured logger such that we always print the version field:
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//
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// ctx = WithLogger(ctx, GetLogger(ctx, "version"))
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//
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// Since the logger has been pushed to the context, we can now get the version
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// field for free with our log messages. Future calls to GetLogger on the new
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// context will have the version field:
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//
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// GetLogger(ctx).Infof("this log message has a version field")
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//
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// This becomes more powerful when we start stacking loggers. Let's say we
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// have the version logger from above but also want a request id. Using the
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// context above, in our request scoped function, we place another logger in
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// the context:
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//
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// ctx = context.WithValue(ctx, "http.request.id", "unique id") // called when building request context
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// ctx = WithLogger(ctx, GetLogger(ctx, "http.request.id"))
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//
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// When GetLogger is called on the new context, "http.request.id" will be
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// included as a logger field, along with the original "version" field:
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//
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// INFO[0000] this log message has a version field http.request.id=unique id version=v2.0.0-alpha.2.m
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//
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// Note that this only affects the new context, the previous context, with the
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// version field, can be used independently. Put another way, the new logger,
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// added to the request context, is unique to that context and can have
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// request scoped variables.
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//
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// HTTP Requests
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//
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// This package also contains several methods for working with http requests.
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// The concepts are very similar to those described above. We simply place the
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// request in the context using WithRequest. This makes the request variables
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// available. GetRequestLogger can then be called to get request specific
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// variables in a log line:
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//
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// ctx = WithRequest(ctx, req)
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// GetRequestLogger(ctx).Infof("request variables")
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//
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// Like above, if we want to include the request data in all log messages in
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// the context, we push the logger to a new context and use that one:
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//
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// ctx = WithLogger(ctx, GetRequestLogger(ctx))
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//
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// The concept is fairly powerful and ensures that calls throughout the stack
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// can be traced in log messages. Using the fields like "http.request.id", one
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// can analyze call flow for a particular request with a simple grep of the
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// logs.
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package context
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