# Node.js startup snapshot builder

This is the V8 startup snapshot builder of Node.js. Not to be confused with
V8's own snapshot builder, which builds a snapshot containing JavaScript
builtins, this builds a snapshot containing Node.js builtins that can be
deserialized on top of V8's own startup snapshot. When Node.js is launched,
instead of executing code to bootstrap, it can deserialize the context from
an embedded snapshot, which readily contains the result of the bootstrap, so
that Node.js can start up faster.

Currently only the main context of the main Node.js instance supports snapshot
deserialization, and the snapshot does not yet cover the entire bootstrap
process. Work is being done to expand the support.

## How it's built and used

The snapshot builder is built with the `node_mksnapshot` target in `node.gyp`
when `node_use_node_snapshot` is set to true, which is currently done by
default.

In the default build of the Node.js executable, to embed a V8 startup snapshot
into the Node.js executable, `libnode` is first built with these unresolved
symbols:

- `node::NodeMainInstance::GetEmbeddedSnapshotBlob`
- `node::NodeMainInstance::GetIsolateDataIndexes`

Then the `node_mksnapshot` executable is built with C++ files in this
directory, as well as `src/node_snapshot_stub.cc` which defines the unresolved
symbols.

`node_mksnapshot` is run to generate a C++ file
`<(SHARED_INTERMEDIATE_DIR)/node_snapshot.cc` that is similar to
`src/node_snapshot_stub.cc` in structure, but contains the snapshot data
written as static char array literals. Then `libnode` is built with
`node_snapshot.cc` to produce the final Node.js executable with the snapshot
data embedded.

For debugging, Node.js can be built without Node.js's own snapshot if
`--without-node-snapshot` is passed to `configure`. A Node.js executable
with Node.js snapshot embedded can also be launched without deserializing
from it if the command line argument `--no-node-snapshot` is passed.