Client/server implementation

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The Bazel system is implemented as a long-lived server process. This allows it to perform many optimizations not possible with a batch-oriented implementation, such as caching of BUILD files, dependency graphs, and other metadata from one build to the next. This improves the speed of incremental builds, and allows different commands, such as build and query to share the same cache of loaded packages, making queries very fast.

When you run bazel, you're running the client. The client finds the server based on the output base, which by default is determined by the path of the base workspace directory and your userid, so if you build in multiple workspaces, you'll have multiple output bases and thus multiple Bazel server processes. Multiple users on the same workstation can build concurrently in the same workspace because their output bases will differ (different userids). If the client cannot find a running server instance, it starts a new one. The server process will stop after a period of inactivity (3 hours, by default, which can be modified using the startup option --max_idle_secs).

For the most part, the fact that there is a server running is invisible to the user, but sometimes it helps to bear this in mind. For example, if you're running scripts that perform a lot of automated builds in different directories, it's important to ensure that you don't accumulate a lot of idle servers; you can do this by explicitly shutting them down when you're finished with them, or by specifying a short timeout period.

The name of a Bazel server process appears in the output of ps x or ps -e f as bazel(dirname), where dirname is the basename of the directory enclosing the root of your workspace directory. For example:

ps -e f
16143 ?        Sl     3:00 bazel(src-johndoe2) -server -Djava.library.path=...

This makes it easier to find out which server process belongs to a given workspace. (Beware that with certain other options to ps, Bazel server processes may be named just java.) Bazel servers can be stopped using the shutdown command.

When running bazel, the client first checks that the server is the appropriate version; if not, the server is stopped and a new one started. This ensures that the use of a long-running server process doesn't interfere with proper versioning.