4810c22607
part of #6628 Before this PR, we used a std::sync::RwLock to coalesce multiple callers on one walredo spawning. One thread would win the write lock and others would queue up either at the read() or write() lock call. In a scenario where a compute initiates multiple getpage requests from different Postgres backends (= different page_service conns), and we don't have a walredo process around, this means all these page_service handler tasks will enter the spawning code path, one of them will do the spawning, and the others will stall their respective executor thread because they do a blocking read()/write() lock call. I don't know exactly how bad the impact is in reality because posix_spawn uses CLONE_VFORK under the hood, which means that the entire parent process stalls anyway until the child does `exec`, which in turn resumes the parent. But, anyway, we won't know until we fix this issue. And, there's definitely a future way out of stalling the pageserver on posix_spawn, namely, forking template walredo processes that fork again when they need to be per-tenant. This idea is tracked in https://github.com/neondatabase/neon/issues/7320. Changes ------- This PR fixes that scenario by switching to use `heavier_once_cell` for coalescing. There is a comment on the struct field that explains it in a bit more nuance. ### Alternative Design An alternative would be to use tokio::sync::RwLock. I did this in the first commit in this PR branch, before switching to `heavier_once_cell`. Performance ----------- I re-ran the `bench_walredo` and updated the results, showing that the changes are neglible. For the record, the earlier commit in this PR branch that uses `tokio::sync::RwLock` also has updated benchmark numbers, and the results / kinds of tiny regression were equivalent to `heavier_once_cell`. Note that the above doesn't measure performance on the cold path, i.e., when we need to launch the process and coalesce. We don't have a benchmark for that, and I don't expect any significant changes. We have metrics and we log spawn latency, so, we can monitor it in staging & prod. Risks ----- As "usual", replacing a std::sync primitive with something that yields to the executor risks exposing concurrency that was previously implicitly limited to the number of executor threads. This would be the first one for walredo. The risk is that we get descheduled while the reconstruct data is already there. That could pile up reconstruct data. In practice, I think the risk is low because once we get scheduled again, we'll likely have a walredo process ready, and there is no further await point until walredo is complete and the reconstruct data has been dropped. This will change with async walredo PR #6548, and I'm well aware of it in that PR.
Neon
Neon is a serverless open-source alternative to AWS Aurora Postgres. It separates storage and compute and substitutes the PostgreSQL storage layer by redistributing data across a cluster of nodes.
Quick start
Try the Neon Free Tier to create a serverless Postgres instance. Then connect to it with your preferred Postgres client (psql, dbeaver, etc) or use the online SQL Editor. See Connect from any application for connection instructions.
Alternatively, compile and run the project locally.
Architecture overview
A Neon installation consists of compute nodes and the Neon storage engine. Compute nodes are stateless PostgreSQL nodes backed by the Neon storage engine.
The Neon storage engine consists of two major components:
- Pageserver: Scalable storage backend for the compute nodes.
- Safekeepers: The safekeepers form a redundant WAL service that received WAL from the compute node, and stores it durably until it has been processed by the pageserver and uploaded to cloud storage.
See developer documentation in SUMMARY.md for more information.
Running local installation
Installing dependencies on Linux
- Install build dependencies and other applicable packages
- On Ubuntu or Debian, this set of packages should be sufficient to build the code:
apt install build-essential libtool libreadline-dev zlib1g-dev flex bison libseccomp-dev \
libssl-dev clang pkg-config libpq-dev cmake postgresql-client protobuf-compiler \
libcurl4-openssl-dev openssl python3-poetry lsof libicu-dev
- On Fedora, these packages are needed:
dnf install flex bison readline-devel zlib-devel openssl-devel \
libseccomp-devel perl clang cmake postgresql postgresql-contrib protobuf-compiler \
protobuf-devel libcurl-devel openssl poetry lsof libicu-devel libpq-devel python3-devel \
libffi-devel
- On Arch based systems, these packages are needed:
pacman -S base-devel readline zlib libseccomp openssl clang \
postgresql-libs cmake postgresql protobuf curl lsof
Building Neon requires 3.15+ version of protoc (protobuf-compiler). If your distribution provides an older version, you can install a newer version from here.
# recommended approach from https://www.rust-lang.org/tools/install
curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
Installing dependencies on macOS (12.3.1)
- Install XCode and dependencies
xcode-select --install
brew install protobuf openssl flex bison icu4c pkg-config
# add openssl to PATH, required for ed25519 keys generation in neon_local
echo 'export PATH="$(brew --prefix openssl)/bin:$PATH"' >> ~/.zshrc
# recommended approach from https://www.rust-lang.org/tools/install
curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
- Install PostgreSQL Client
# from https://stackoverflow.com/questions/44654216/correct-way-to-install-psql-without-full-postgres-on-macos
brew install libpq
brew link --force libpq
Rustc version
The project uses rust toolchain file to define the version it's built with in CI for testing and local builds.
This file is automatically picked up by rustup that installs (if absent) and uses the toolchain version pinned in the file.
rustup users who want to build with another toolchain can use the rustup override command to set a specific toolchain for the project's directory.
non-rustup users most probably are not getting the same toolchain automatically from the file, so are responsible to manually verify that their toolchain matches the version in the file. Newer rustc versions most probably will work fine, yet older ones might not be supported due to some new features used by the project or the crates.
Building on Linux
- Build neon and patched postgres
# Note: The path to the neon sources can not contain a space.
git clone --recursive https://github.com/neondatabase/neon.git
cd neon
# The preferred and default is to make a debug build. This will create a
# demonstrably slower build than a release build. For a release build,
# use "BUILD_TYPE=release make -j`nproc` -s"
# Remove -s for the verbose build log
make -j`nproc` -s
Building on OSX
- Build neon and patched postgres
# Note: The path to the neon sources can not contain a space.
git clone --recursive https://github.com/neondatabase/neon.git
cd neon
# The preferred and default is to make a debug build. This will create a
# demonstrably slower build than a release build. For a release build,
# use "BUILD_TYPE=release make -j`sysctl -n hw.logicalcpu` -s"
# Remove -s for the verbose build log
make -j`sysctl -n hw.logicalcpu` -s
Dependency installation notes
To run the psql client, install the postgresql-client package or modify PATH and LD_LIBRARY_PATH to include pg_install/bin and pg_install/lib, respectively.
To run the integration tests or Python scripts (not required to use the code), install
Python (3.9 or higher), and install the python3 packages using ./scripts/pysync (requires poetry>=1.3) in the project directory.
Running neon database
- Start pageserver and postgres on top of it (should be called from repo root):
# Create repository in .neon with proper paths to binaries and data
# Later that would be responsibility of a package install script
> cargo neon init
Initializing pageserver node 1 at '127.0.0.1:64000' in ".neon"
# start pageserver, safekeeper, and broker for their intercommunication
> cargo neon start
Starting neon broker at 127.0.0.1:50051.
storage_broker started, pid: 2918372
Starting pageserver node 1 at '127.0.0.1:64000' in ".neon".
pageserver started, pid: 2918386
Starting safekeeper at '127.0.0.1:5454' in '.neon/safekeepers/sk1'.
safekeeper 1 started, pid: 2918437
# create initial tenant and use it as a default for every future neon_local invocation
> cargo neon tenant create --set-default
tenant 9ef87a5bf0d92544f6fafeeb3239695c successfully created on the pageserver
Created an initial timeline 'de200bd42b49cc1814412c7e592dd6e9' at Lsn 0/16B5A50 for tenant: 9ef87a5bf0d92544f6fafeeb3239695c
Setting tenant 9ef87a5bf0d92544f6fafeeb3239695c as a default one
# create postgres compute node
> cargo neon endpoint create main
# start postgres compute node
> cargo neon endpoint start main
Starting new endpoint main (PostgreSQL v14) on timeline de200bd42b49cc1814412c7e592dd6e9 ...
Starting postgres at 'postgresql://cloud_admin@127.0.0.1:55432/postgres'
# check list of running postgres instances
> cargo neon endpoint list
ENDPOINT ADDRESS TIMELINE BRANCH NAME LSN STATUS
main 127.0.0.1:55432 de200bd42b49cc1814412c7e592dd6e9 main 0/16B5BA8 running
- Now, it is possible to connect to postgres and run some queries:
> psql -p 55432 -h 127.0.0.1 -U cloud_admin postgres
postgres=# CREATE TABLE t(key int primary key, value text);
CREATE TABLE
postgres=# insert into t values(1,1);
INSERT 0 1
postgres=# select * from t;
key | value
-----+-------
1 | 1
(1 row)
- And create branches and run postgres on them:
# create branch named migration_check
> cargo neon timeline branch --branch-name migration_check
Created timeline 'b3b863fa45fa9e57e615f9f2d944e601' at Lsn 0/16F9A00 for tenant: 9ef87a5bf0d92544f6fafeeb3239695c. Ancestor timeline: 'main'
# check branches tree
> cargo neon timeline list
(L) main [de200bd42b49cc1814412c7e592dd6e9]
(L) ┗━ @0/16F9A00: migration_check [b3b863fa45fa9e57e615f9f2d944e601]
# create postgres on that branch
> cargo neon endpoint create migration_check --branch-name migration_check
# start postgres on that branch
> cargo neon endpoint start migration_check
Starting new endpoint migration_check (PostgreSQL v14) on timeline b3b863fa45fa9e57e615f9f2d944e601 ...
Starting postgres at 'postgresql://cloud_admin@127.0.0.1:55434/postgres'
# check the new list of running postgres instances
> cargo neon endpoint list
ENDPOINT ADDRESS TIMELINE BRANCH NAME LSN STATUS
main 127.0.0.1:55432 de200bd42b49cc1814412c7e592dd6e9 main 0/16F9A38 running
migration_check 127.0.0.1:55434 b3b863fa45fa9e57e615f9f2d944e601 migration_check 0/16F9A70 running
# this new postgres instance will have all the data from 'main' postgres,
# but all modifications would not affect data in original postgres
> psql -p 55434 -h 127.0.0.1 -U cloud_admin postgres
postgres=# select * from t;
key | value
-----+-------
1 | 1
(1 row)
postgres=# insert into t values(2,2);
INSERT 0 1
# check that the new change doesn't affect the 'main' postgres
> psql -p 55432 -h 127.0.0.1 -U cloud_admin postgres
postgres=# select * from t;
key | value
-----+-------
1 | 1
(1 row)
- If you want to run tests afterwards (see below), you must stop all the running pageserver, safekeeper, and postgres instances you have just started. You can terminate them all with one command:
> cargo neon stop
More advanced usages can be found at Control Plane and Neon Local.
Handling build failures
If you encounter errors during setting up the initial tenant, it's best to stop everything (cargo neon stop) and remove the .neon directory. Then fix the problems, and start the setup again.
Running tests
Rust unit tests
We are using cargo-nextest to run the tests in Github Workflows.
Some crates do not support running plain cargo test anymore, prefer cargo nextest run instead.
You can install cargo-nextest with cargo install cargo-nextest.
Integration tests
Ensure your dependencies are installed as described here.
git clone --recursive https://github.com/neondatabase/neon.git
CARGO_BUILD_FLAGS="--features=testing" make
./scripts/pytest
By default, this runs both debug and release modes, and all supported postgres versions. When testing locally, it is convenient to run just one set of permutations, like this:
DEFAULT_PG_VERSION=15 BUILD_TYPE=release ./scripts/pytest
Flamegraphs
You may find yourself in need of flamegraphs for software in this repository.
You can use flamegraph-rs or the original flamegraph.pl. Your choice!
Important
If you're using
lldormold, you need the--no-rosegmentlinker argument. It's a general thing with Rust / lld / mold, not specific to this repository. See this PR for further instructions.
Cleanup
For cleaning up the source tree from build artifacts, run make clean in the source directory.
For removing every artifact from build and configure steps, run make distclean, and also consider removing the cargo binaries in the target directory, as well as the database in the .neon directory. Note that removing the .neon directory will remove your database, with all data in it. You have been warned!
Documentation
docs Contains a top-level overview of all available markdown documentation.
- sourcetree.md contains overview of source tree layout.
To view your rustdoc documentation in a browser, try running cargo doc --no-deps --open
See also README files in some source directories, and rustdoc style documentation comments.
Other resources:
- SELECT 'Hello, World': Blog post by Nikita Shamgunov on the high level architecture
- Architecture decisions in Neon: Blog post by Heikki Linnakangas
- Neon: Serverless PostgreSQL!: Presentation on storage system by Heikki Linnakangas in the CMU Database Group seminar series
Postgres-specific terms
Due to Neon's very close relation with PostgreSQL internals, numerous specific terms are used. The same applies to certain spelling: i.e. we use MB to denote 1024 * 1024 bytes, while MiB would be technically more correct, it's inconsistent with what PostgreSQL code and its documentation use.
To get more familiar with this aspect, refer to:
- Neon glossary
- PostgreSQL glossary
- Other PostgreSQL documentation and sources (Neon fork sources can be found here)
Join the development
- Read CONTRIBUTING.md to learn about project code style and practices.
- To get familiar with a source tree layout, use sourcetree.md.
- To learn more about PostgreSQL internals, check http://www.interdb.jp/pg/index.html