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Live Update Reference

(This doc provides the technical specifications of Tilt’s LiveUpdate functionality. For a tutorial that walks you through a sample project, see Faster Development with Live Update (Tutorial).)

When specifying how to build an image (via docker_build() or custom_build()), you may optionally pass the live_update argument.

live_update takes a list of LiveUpdateSteps that tell Tilt how to update a running container in place (instead of paying the cost of building a new image and redeploying).

The list of LiveUpdateSteps must be, in order:

When you tilt up, your initial build will be a full build—i.e., the specified Docker build or Custom build.1

When a file changes:

  1. If it matches any of the files in a fall_back_on step, we will fall back to a full rebuild + deploy (i.e. the normal, non-live_update process).
  2. Otherwise, if it matches any of the local paths in sync steps, a live update will be executed as follows:
    1. copy any changed files according to sync steps
    2. for every run step:
      1. if the run specifies one or more triggers, execute the command iff any changed files match the given triggers
      2. otherwise, simply execute the command

LiveUpdateSteps

Each of the functions above returns a LiveUpdateStep – an object like any other, i.e. it can be assigned to a variable, etc. That means that something like this is perfectly valid syntax:

sync_src = sync('./source', '/app/src')
sync_static = sync('./static', '/app/static')
docker_build('my-img', '.', live_update=[sync_src, sync_static])

As part of Tiltfile validation, we check that all of the LiveUpdateSteps you’ve created have been used in at least one Live Update call. If not, we throw an error.

sync(local_path: str, remote_path: str)

sync steps are the backbone of a Live Update. (For this reason, we’ll discuss them first, even though they may be preceeded by one or more fall_back_on steps in a Tiltfile.)

Tilt will only run a Live Update if it detects a change to one or more files matching a sync step.

A sync call takes two args: the local path of a file/directory, and the remote path to which Tilt should sync that file/directory if it changes. (This includes deleting the file remotely if it is deleted locally.)

What files can I sync? How are builds triggered?

When you tell Tilt how to build an image, you specify some set of files to watch. In the case of a docker_build call, Tilt watches the directory that you pass as context. Your sync’d local paths must fall within that context. (If you’re using custom_build, all of the above applies, only with deps in place of context.) Let’s look at some examples:

An illegal 'sync'
An illegal 'sync': attempting to sync files that aren't included in the docker_build context

The sync above is invalid, because it attempts to sync files that we’re not even watching (seen here highlighted in blue). To put it another way: there’s no way for those files to get into the container in the first place, because they would never be included in the Docker build. (If this is functionality you need, let us know.)

A valid use of 'sync' (all sync'd files are subsets of docker_build.context)
A valid use of 'sync' (all sync'd files are subsets of docker_build.context)

Above is an example of a valid syncs. A change to any of the green files will kick off a Live Update, because they match a sync step. A change to any of the yellow files will kick off a full Docker build + deploy, because they’re part of the Docker context but we don’t have instructions on how to Live Update them. (Want to be more selective in which files do/don’t kick off full builds? Check out context filters for docker_build.)

How to use 'sync' with multiple dependent docker images
An example of 'sync' used with dependent Docker images

If you have multiple Docker images that depend on each other, you can sync files from anywhere within the contexts of any of the images. (In the diagram above, Tilt is building two images; the yellow image in the server1 directory depends on–i.e. FROM’s–the red image in the common directory.)

The rule of thumb is: if Tilt is watching it, you can sync it. (Tilt will watch it if it’s in a docker_build.context or custom_build.deps).

Let’s review

For instance, take the following sync statements:

docker_build('my-img', './server', live_update=[
    sync('./server/src', '/app'),
    sync('./server/package.json', '/app/web/package.json')
])
  1. change to ./server/src/A.txt => synced to /app/A.txt
  2. change to ./server/package.json => synced to /app/web/package.json
  3. change to ./server/some_file.txt => doesn’t match any sync statements, but does match the docker_build context; instead of a Live Update, Tilt performs a Docker Build
  4. change to ./stuff.txt => doesn’t match a sync statement OR the docker_build context; nothing happens

fall_back_on(files: str || List[str])

This step is optional, though if provided, it must come at the beginning of the live_update call. The argument is a filepath (string) or multiple filepaths (list of strings) on your local machine, either absolute or relative to the Tiltfile. Whenever Tilt detects a change to your local filesystem that would otherwise trigger a LiveUpdate, it first checks if it matches any fall_back_on files; if yes, instead of doing a LiveUpdate, Tilt falls back to a full rebuild + deploy.

run(cmd: str, trigger=None: str || List[str])

The first argument to run is a the command to be executed on the running container. Currently, all commands are executed from /, so be sure to use absolute paths!

Currently, all of your run steps must come after all of your sync steps. (If you need different behavior, let us know!) If you provide multiple run steps, the commands will be executed in the order given.

Run triggers

The second arg, trigger, is optional. If you don’t provide a trigger, then the command is executed on the container whenever Tilt performs a LiveUpdate (i.e. whenever Tilt detects a change to one or more files matching a sync).

A trigger is a filepath (string) or multiple filepaths (list of strings) on your local machine, either absolute or relative to the Tiltfile. If a trigger is provided, Tilt only executes the command if a changed file matches one the trigger paths.

Note that specifying a file as a trigger does not tell Tilt to watch and/or sync that file; that is, all trigger files must also be included in a sync step.

Let’s walk through what will and won’t happen when various files change, given this example Tiltfile:

docker_build('my-img', '.', live_update=[
    sync('./src', '/app'),
    run('/app/setup.sh'),
    run('cd /app/web && yarn install', trigger='./src/web/yarn.lock'),
    run('/app/run_configs.sh', trigger=['./configs/foo.yaml', './configs/bar.yaml'])
])
  1. change to ./src/main.py => this file matches a sync step, so we run the Live Update. We run setup.sh, because it has no triggers and so runs on every Live Update. We run neither of the other run steps, b/c this file doesn’t match any of their triggers.
  2. change to ./src/web/yarn.lock => run the Live Update; run setup.sh; run cd /app/web && yarn install, because this file matches that command’s trigger
  3. change to ./configs/foo.yaml => whoops, this file doesn’t match any sync steps! Even though it matches a trigger (for the third run), we won’t do a Live Update for this change; instead, we do a full Docker build (see notes on sync, above, for what changes trigger a Live Update vs. a full build + deploy).

Restarting your Process

Some apps or invocations thereof (e.g. Javascript apps run via nodemon, or Flask apps run in debug mode) detect and incorporate code changes without needing to restart. For other apps, though, you’ll need to re-execute them for changes to take effect.

For most setups, you’ll be able to use the restart_process extension: import the extension, replace your docker_build call with a docker_build_with_restart call, and specify the entrypoint parameter (i.e. the command to run on container start and re-run on Live Update).

There are a few exceptions to the above; the restart_process extension doesn’t work for:

  • Docker Compose resources; you should use the restart_container() Live Update step instead
  • Images build via custom_build
  • Container images without a shell (e.g. scratch, distroless)

If any of the exceptions above apply to you, or restart_process doesn’t otherwise work for your use case, read on.

Workarounds for Restarting Your Process

Tilt is flexible enough that you can employ any number of workarounds for restarting your process as part of a Live Update. The basic idea is to invoke your process such that a single command (specified as a run step) causes it to restart. Here are a few approaches we recommend:

  • We’ve written a set of wrappers for your process. Put these scripts in your container and invoke your process as:
    /path/to/start.sh /path/to/bin
    

    You can then restart your process with Live Update step: run('/path/to/restart.sh'). (Requires that shell be available on your container.)

  • entr is a neat utility for (re)running processes when specified files change. You can designate an arbitrary file to trigger process restart, say /restart.txt, and invoke your process like this:
    echo /restart.txt | entr -rz /path/to/bin
    

    You can then your process with Live Update step: run('date > /restart.txt'). (You’ll have to ensure that entr is present in your Docker image, and that your arbitrary file for restarting exists.) (Requires that shell be available on your container.)

Recall that you can change the command run by your container in a few ways:

  • in the Dockerfile, via ENTRYPOINT/CMD
  • in your Kubernetes YAML, via spec.containers.[the_container].command
  • in your Tiltfile, via the docker_build.entrypoint parameter (or analogously, custom_build.entrypoint)

More Examples

If you need more specifics on how to set up Live Update with your programming language, all our major example projects use Live Update:


  1. The initial build is always a full build because Live Update needs a running container to modify. Thus, your base build (Docker/Custom Build) should be sufficient to create your dev image, and should not rely on any sync‘d files or run commands. (If this is functionality you need, let us know.) 

 
 
 

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