For your library you may commit the composer.lock file if you want to. This can help your team to always test against the same dependency versions. However, this lock file will not have any effect on other projects that depend on it. It only has an effect on the main project.
composer.lock is build metadata which is not part of the project. The state of dependencies should be controlled through how you're versioning them (either manually or as part of your automated build process) and not arbitrarily by the last developer to update them and commit the lock file.
If you are concerned about your dependencies changing between composer updates then you have a lack of confidence in your versioning scheme. Versions (1.0, 1.1, 1.2, etc) should be immutable and you should avoid "dev-" and "X.*" wildcards outside of initial feature development.
Also, your project should never have to be rebuilt or have its dependencies reacquired in each environment, especially prod. Your deliverable (tar, zip, phar, a directory, etc) should be immutable and promoted through environments without changing.
1. You shouldn't update your dependencies directly on Production, because you don't know how this will affect the stability of your code. There could be bugs introduced with the new dependencies, it might change the way the code behaves affecting your own, it could be incompatible with other dependencies, etc. You should do this in a dev environment, following by proper QA and regression testing, etc.
2. You should version control your composer.lock file, because this stores information about your dependencies and about the dependencies of your dependencies that will allow you to replicate the current state of the code. This is important, because, all your testing and development has been done against specific code. Not caring about the actual version of the code that you have is similar to uploading code changes to your application and not testing them. If you are upgrading your dependencies versions, this should be a willingly act, and you should take the necessary care to make sure everything still works. Losing one or two hours of up time reverting to a previous release version might cost you a lot of money.
One of the arguments that you will see about not needing the composer.lock is that you can set the exact version that you need in your composer.json file, and that in this way, every time someone runs composer install, it will install them the same code. This is not true, because, your dependencies have their own dependencies, and their configuration might be specified in a format that it allows updates to subversions, or maybe even entire versions.
This means that even when you specify that you want Laravel 4.1.31 in your composer.json, Laravel in its composer.json file might have its own dependencies required as Symfony event-dispatcher: 2.*. With this kind of config, you could end up with Laravel 4.1.31 with Symfony event-dispatcher 2.4.1, and someone else on your team could have Laravel 4.1.31 with event-dispatcher 2.6.5, it would all depend on when was the last time you ran the composer install.
So, having your composer.lock file in the version system will store the exact version of this sub-dependencies, so, when you and your teammate does a composer install (this is the way that you will install your dependencies based on a composer.lock) you both will get the same versions.
What if you wanna update? Then in your dev environment run: composer update, this will generate a new composer.lock file (if there is something new) and after you test it, and QA test and regression test it and stuff. You can push it for everyone else to download the new composer.lock, since its safe to upgrade.
3. You shouldn't version control your actual dependencies, because it makes no sense. With the composer.lock you can install the exact version of the dependencies and you wouldn't need to commit them. Why would you add to your repo 10000 files of dependencies, when you are not supposed to be updating them. If you require to change one of this, you should fork it and make your changes there. And if you are worried about having to fetch the actual dependencies each time of a build or release, composer has different ways to alleviate this issue, cache, zip files, etc.
The point of the lock file is to record the exact versions that are installed so they can be re-installed. This means that if you have a version spec of 1.* and your co-worker runs composer update which installs 1.2.4, and then commits the composer.lock file, when you composer install, you will also get 1.2.4, even if 1.3.0 has been released. This ensures everybody working on the project has the same exact version.
Commit your application's composer.lock (along with composer.json) into version control. This is important because the install command checks if a lock file is present, and if it is, it downloads the versions specified there (regardless of what composer.json says). This means that anyone who sets up the project will download the exact same version of the dependencies. Your CI server, production machines, other developers in your team, everything and everyone runs on the same dependencies, which mitigates the potential for bugs affecting only some parts of the deployments. Even if you develop alone, in six months when reinstalling the project you can feel confident the dependencies installed are still working even if your dependencies released many new versions since then.
The exception is when you use a meta apps, libraries where the dependencies should be updated on install (like the Zend Framework 2 Skeleton App). So the aim is to grab the latest dependencies each time when you want to start developing.
The file composer.lock maintains dependency in more depth i.e it points to the actual commit of the version of the package we include in our software, hence this is one of the most important files which handles the dependency more finely.
its worth to mention one of the 12 factor modern web development principles is to keep the dependencies the same along all deploys, so every deploy can test it.and there would be no contraction between them.
Push composer.lock and use composer install in production.
Don't commit changes to composer.lock unless a big test effort is prepared in preprod.
composer install: will use existing composer.lock versions
composer update: will update to newest versions and modify composer.lock
You want to always use composer.lock via composer install in prod so you have tested the update in preprod (a lot of testing).
If you don't push any composer.lock you risk newer versions of your dependencies breaking your prod.
Developers should also do composer install when composer.lock is present unless your objective is to update the dependencies and have planned testing in preprod, meaning, don't commit it unless you are planning to do a big test effort.If a dev innadvertedly made changes to it locally, revert those changes and do composer install to revert to the prod versions.
Generally speaking, composer shouldn't be doing what the build system is meant to be doing and you shouldn't be putting composer.lock in VCS. Composer might strangely have it backwards. End users rather than produces shouldn't be using lock files. Usually your build system keeps snapshots, reusable dirs, etc rather than an empty dir each time. People checkout out a lib from composer might want that lib to use a lock so that the dependencies that lib loads have been tested against.
On the other hand that significantly increases the burden of version management, where you'd almost certainly want multiple versions of every library as dependencies will be strictly locked. If every library is likely to have a slightly different version then you need some multiple library version support and you can also quickly see the size of dependencies needed flair out, hence the advise to keep it on the leaf.
Taking that on board, I really don't find lock files to be useful either libraries or your own workdirs. It's only use for me is in my build/testing platform which persists any externally acquired assets only updating them when requested, providing repeatable builds for testing, build and deploy. While that can be kept in VCS it's not always kept with the source tree, the build trees will either be elsewhere in the VCS structure or managed by another system somewhere else. If it's stored in a VCS it's debatable whether or not to keep it in the same repo as source trees because otherwise every pull can bring in a mass of build assets. I quite like having things all in a well arranged repo with the exception of production/sensitive credentials and bloat.
SVN can do it better than git as it doesn't force you to acquire the entire repo (though I suspect that's not actually strictly needed for git either but support for that is limited and it's not commonly used). Simple build repos are usually just an overlay branch you merge/export the build tree into. Some people combine exernal resources in their source tree or separate further, external, build and source trees. It usually serves two purposes, build caching and repeatable builds but sometimes keeping it separate on at least some level also permits fresh/blank builds and multiple builds easily.
The arguments people are putting forward for lock files are cases where they've taken a very specific and restrictive view of the problem. Want repeatable builds and consistent builds? Include the vendor folder in VCS. Then you also speed up fetching assets as well as not having to depend on potentially broken external resources during build. None of the build and deploy pipelines I create require external access unless absolutely necessary. If you do have to update an external resource it's once and only once. What composer is trying to achieve makes sense for a distributed system except as mentioned before it makes no sense because it would end up with library dependency hell for library updates with common clashes and updates being as slow as the slowest to update package.
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