1# Synthtool and Autosynth Protocol 2 3**Synthtool** generates client library source code according to the currrent 4state of the source repo, and upstream repos like 5[googleapis/googleapis](https://github.com/googleapis/googleapis). 6 7**Autosynth** periodically runs Synthtool for each client library source repo. 8Additionally, it repeatedly runs Synthtool in a binary search pattern to 9discover which upstream change triggered a change in the generate code. 10 11The proto files in this directory define the interface between. 12 13 14 15**Synthtool** generates a file called `synth.metadata`, which contains a 16json-encoded instance of the `Metadata` class defined in [metadata.proto](./metadata.proto). `synth.metadata` tells Autosynth which git repos were pulled 17by Synthtool while generating the library's source code. 18 19**Autosynth** consumes `synth.metadata`. Before invoking Synthtool again, it 20observes which repos Synthtool used during the last generation, and 21fetches them in advance. Autosynth tells Synthtool where to find those repos 22by generating a `preconfig.json` file. 23 24The `preconfig.json` file contains a Preconfig message as defined in 25[preconfig.proto](./preconfig.proto). Autosynth sets the environment variable 26`SYNTHTOOL_PRECONFIG_FILE` to the path of the `preconfig.json` file so that 27Synthtool can find it. 28 29Here's a sample `preconfig.json` file: 30``` 31{ 32 "preclonedRepos": { 33 "https://github.com/googleapis/google-cloud-dotnet.git": "/tmpfs/src/git/autosynth/working_repo", 34 "https://github.com/googleapis/googleapis.git": "/tmpfs/tmp/tmp4pusokpm/googleapis" 35 } 36} 37```
