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view mercurial/help/scripting.txt @ 26644:74de1c59f71c
clonebundles: filter on bundle specification
Not all clients are capable of reading every bundle. Currently, content
negotiation to ensure a server sends a client a compatible bundle
format is performed at request time. The response bundle is dynamically
generated at request time, so this works fine.
Clone bundles are statically generated *before* the request. This means
that a modern server could produce bundles that a legacy client isn't
capable of reading. Without some kind of "type hint" in the clone
bundles manifest, a client may attempt to download an incompatible
bundle. Furthermore, a client may not realize a bundle is incompatible
until it has processed part of the bundle (imagine consuming a 1 GB
changegroup bundle2 part only to discover the bundle2 part afterwards is
incompatibl). This would waste time and resources. And it isn't very
user friendly.
Clone bundle manifests thus need to advertise the *exact* format of the
hosted bundles so clients may filter out entries that they don't know
how to read. This patch introduces that mechanism.
We introduce the BUNDLESPEC attribute to declare the "bundle
specification" of the entry. Bundle specifications are parsed using
exchange.parsebundlespecification, which uses the same strings as the
"--type" argument to `hg bundle`. The supported bundle specifications
are well defined and backwards compatible.
When a client encounters a BUNDLESPEC that is invalid or unsupported, it
silently ignores the entry.
author | Gregory Szorc <gregory.szorc@gmail.com> |
---|---|
date | Tue, 13 Oct 2015 11:45:30 -0700 |
parents | c38afb8c7deb |
children | 7ce05671a5e3 |
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It is common for machines (as opposed to humans) to consume Mercurial. This help topic describes some of the considerations for interfacing machines with Mercurial. Choosing an Interface ===================== Machines have a choice of several methods to interface with Mercurial. These include: - Executing the ``hg`` process - Querying a HTTP server - Calling out to a command server Executing ``hg`` processes is very similar to how humans interact with Mercurial in the shell. It should already be familiar to you. :hg:`serve` can be used to start a server. By default, this will start a "hgweb" HTTP server. This HTTP server has support for machine-readable output, such as JSON. For more, see :hg:`help hgweb`. :hg:`serve` can also start a "command server." Clients can connect to this server and issue Mercurial commands over a special protocol. For more details on the command server, including links to client libraries, see https://mercurial.selenic.com/wiki/CommandServer. :hg:`serve` based interfaces (the hgweb and command servers) have the advantage over simple ``hg`` process invocations in that they are likely more efficient. This is because there is significant overhead to spawn new Python processes. .. tip:: If you need to invoke several ``hg`` processes in short order and/or performance is important to you, use of a server-based interface is highly recommended. Environment Variables ===================== As documented in :hg:`help environment`, various environment variables influence the operation of Mercurial. The following are particularly relevant for machines consuming Mercurial: HGPLAIN If not set, Mercurial's output could be influenced by configuration settings that impact its encoding, verbose mode, localization, etc. It is highly recommended for machines to set this variable when invoking ``hg`` processes. HGENCODING If not set, the locale used by Mercurial will be detected from the environment. If the determined locale does not support display of certain characters, Mercurial may render these character sequences incorrectly (often by using "?" as a placeholder for invalid characters in the current locale). Explicitly setting this environment variable is a good practice to guarantee consistent results. "utf-8" is a good choice on UNIX-like environments. HGRCPATH If not set, Mercurial will inherit config options from config files using the process described in :hg:`help config`. This includes inheriting user or system-wide config files. When utmost control over the Mercurial configuration is desired, the value of ``HGRCPATH`` can be set to an explicit file with known good configs. In rare cases, the value can be set to an empty file or the null device (often ``/dev/null``) to bypass loading of any user or system config files. Note that these approaches can have unintended consequences, as the user and system config files often define things like the username and extensions that may be required to interface with a repository. Consuming Command Output ======================== It is common for machines to need to parse the output of Mercurial commands for relevant data. This section describes the various techniques for doing so. Parsing Raw Command Output -------------------------- Likely the simplest and most effective solution for consuming command output is to simply invoke ``hg`` commands as you would as a user and parse their output. The output of many commands can easily be parsed with tools like ``grep``, ``sed``, and ``awk``. A potential downside with parsing command output is that the output of commands can change when Mercurial is upgraded. While Mercurial does generally strive for strong backwards compatibility, command output does occasionally change. Having tests for your automated interactions with ``hg`` commands is generally recommended, but is even more important when raw command output parsing is involved. Using Templates to Control Output --------------------------------- Many ``hg`` commands support templatized output via the ``-T/--template`` argument. For more, see :hg:`help templates`. Templates are useful for explicitly controlling output so that you get exactly the data you want formatted how you want it. For example, ``log -T {node}\n`` can be used to print a newline delimited list of changeset nodes instead of a human-tailored output containing authors, dates, descriptions, etc. .. tip:: If parsing raw command output is too complicated, consider using templates to make your life easier. The ``-T/--template`` argument allows specifying pre-defined styles. Mercurial ships with the machine-readable styles ``json`` and ``xml``, which provide JSON and XML output, respectively. These are useful for producing output that is machine readable as-is. .. important:: The ``json`` and ``xml`` styles are considered experimental. While they may be attractive to use for easily obtaining machine-readable output, their behavior may change in subsequent versions. These styles may also exhibit unexpected results when dealing with certain encodings. Mercurial treats things like filenames as a series of bytes and normalizing certain byte sequences to JSON or XML with certain encoding settings can lead to surprises. Command Server Output --------------------- If using the command server to interact with Mercurial, you are likely using an existing library/API that abstracts implementation details of the command server. If so, this interface layer may perform parsing for you, saving you the work of implementing it yourself. Output Verbosity ---------------- Commands often have varying output verbosity, even when machine readable styles are being used (e.g. ``-T json``). Adding ``-v/--verbose`` and ``--debug`` to the command's arguments can increase the amount of data exposed by Mercurial. An alternate way to get the data you need is by explicitly specifying a template. Other Topics ============ revsets Revisions sets is a functional query language for selecting a set of revisions. Think of it as SQL for Mercurial repositories. Revsets are useful for querying repositories for specific data. See :hg:`help revsets` for more. share extension The ``share`` extension provides functionality for sharing repository data across several working copies. It can even automatically "pool" storage for logically related repositories when cloning. Configuring the ``share`` extension can lead to significant resource utilization reduction, particularly around disk space and the network. This is especially true for continuous integration (CI) environments. See :hg:`help -e share` for more.