pearldb

by willemt

willemt / pearldb

A Lightweight Durable HTTP Key-Value Pair Database in C

131 Stars 18 Forks Last release: about 5 years ago (0.1.0) Other 140 Commits 1 Releases

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What?

PearlDB is a durable HTTP Key-Value pair database. It uses

LMDB 
_ for storing data, and
H2O 
_ for HTTP.

PearlDB is completely written in C.

Persistent connections and pipelining are built-in.

PearlDB uses

bmon 
_ to batch LMDB writes.

Goals

  • Speed
  • Low latency
  • Durability - An HTTP response means the write is on disk
  • Simplicity outside (RESTful inteface)
  • Simplicity inside (succinct codebase)
  • HTTP caching - Because the CRUD is RESTful you could hypothetically use an HTTP reverse proxy cache to scale out reads. You could use multiple caches to create an eventually consistent database

Ubuntu Quick Start

.. code-block:: bash :class: ignore

sudo add-apt-repository -y ppa:willemt/pearldb sudo apt-get update sudo apt-get install pearldb

Usage

Examples below make use of the excellent

httpie 
_

Starting

.. code-block:: bash

build/pearl --daemonize --port 8000 --dbsize 1 --pidfile pearl.pid echo daemonizing...

.. code-block:: bash

daemonizing...

Get

We obtain a value by GET'ng the key.

In this case the key is "x". But we get a 404 if it doesn't exist.

.. code-block:: bash

http -h --ignore-stdin 127.0.0.1:8000/x/

.. code-block:: bash :class: dotted

HTTP/1.1 404 NOT FOUND Date: ..., ... .... ........ GMT Connection: keep-alive Server: h2o/2.2.4 content-length: 0

You MUST specify a path.

.. code-block:: bash

http -h --ignore-stdin 127.0.0.1:8000/ | head -n 1

.. code-block:: bash :class: dotted

HTTP/1.1 400 BAD PATH

Put

We use PUT for creating or updating a key value pair. PUTs are

durable 
_ - we only respond when the change has been made to disk.

.. code-block:: bash

echo "MY VALUE" | http -h PUT 127.0.0.1:8000/x/

.. code-block:: bash :class: dotted

HTTP/1.1 200 OK Date: ..., ... .... ........ GMT Connection: keep-alive Server: h2o/2.2.4 transfer-encoding: chunked

PUTs have an immediate change on the resource. There is full

isolation 
, and therefore no `dirty reads <http://en.wikipedia.org/wiki/Isolation(databasesystems)#Dirtyreads>`_.

Now we can finally retrieve our data via a GET:

.. code-block:: bash

http --ignore-stdin 127.0.0.1:8000/x/

.. code-block:: bash

MY VALUE

The slash at the end is optional.

.. code-block:: bash

http --ignore-stdin 127.0.0.1:8000/x

.. code-block:: bash

MY VALUE

The user must specify the capacity of the database upfront. PearlDB does not support automatic resizing. A PUT will fail if it would put the database over capacity.

.. code-block:: bash

head -c 1000000 /dev/urandom | base64 > tmpfile du -h tmpfile | awk '{ print $1 }' cat tmpfile | http -h PUT 127.0.0.1:8000/1/ rm tmpfile

.. code-block:: bash :class: dotted

1.3M HTTP/1.1 400 NOT ENOUGH SPACE Date: ..., ... .... ........ GMT Connection: keep-alive Server: h2o/2.2.4 content-length: 0

You can't PUT under nested resources.

.. code-block:: bash

echo 'DATA' | http -h PUT 127.0.0.1:8000/x/nested_resource/

.. code-block:: bash :class: dotted

HTTP/1.1 400 BAD PATH Date: ..., ... .... ........ GMT Connection: keep-alive Server: h2o/2.2.4 content-length: 0

Put without a key (POST)

If you want PearlDB to generate a key for you, just use POST.

.. code-block:: bash

echo "MY POSTED VALUE" | http -h POST 127.0.0.1:8000/ > posted.txt cat posted.txt

.. code-block:: bash :class: dotted

HTTP/1.1 200 OK Date: ..., ... .... ........ GMT Connection: keep-alive Server: h2o/2.2.4 location: ... transfer-encoding: chunked

The Location header in the response has the URI of the newly created resource. The URI is the URL safe base64 encoded UUID4.

.. code-block:: bash

http --ignore-stdin -b GET 127.0.0.1:8000$(grep location: posted.txt | sed -e 's/location: //' | tr -d '\r\n')

.. code-block:: bash :class: dotted

MY POSTED VALUE

Providing a URL (ie. key) with POST doesn't make sense, and will result in a 400.

.. code-block:: bash

echo "MY POSTED VALUE" | http -h POST 127.0.0.1:8000/xxxx/

.. code-block:: bash :class: dotted

HTTP/1.1 400 BAD Date: ..., ... .... ........ GMT Connection: keep-alive Server: h2o/2.2.4 content-length: 0

Get keys

You can get the keys that match a prefix by using the /key/XXX/ nested resource.

.. code-block:: bash

echo '' | http PUT 127.0.0.1:8000/1/ > /dev/null echo '' | http PUT 127.0.0.1:8000/199/ > /dev/null echo '' | http PUT 127.0.0.1:8000/102/ > /dev/null echo '' | http PUT 127.0.0.1:8000/2/ > /dev/null http GET 127.0.0.1:8000/key/1/

.. code-block:: bash :class: dotted

1 102 199

Without a prefix you get all keys.

.. code-block:: bash

http GET 127.0.0.1:8000/key// | sed -e '/^.*=$/d'

.. code-block:: bash :class: dotted

1 102 199 2 x

Existence Check

To check for existence use the HEAD method. This is great, because PearlDB doesn't waste bandwidth sending the document body.

.. code-block:: bash

http -h --ignore-stdin HEAD 127.0.0.1:8000/x/

.. code-block:: bash :class: dotted

HTTP/1.1 200 OK Date: ..., ... .... ........ GMT Connection: keep-alive Server: h2o/2.2.4

Delete

DELETEs are durable - we only respond when the change has been made to disk.

.. code-block:: bash

http -h --ignore-stdin DELETE 127.0.0.1:8000/x/

.. code-block:: bash :class: dotted

HTTP/1.1 200 OK Date: ..., ... .... ........ GMT Connection: keep-alive Server: h2o/2.2.4 transfer-encoding: chunked

Of course, after a DELETE the key doesn't exist anymore:

.. code-block:: bash

http -h --ignore-stdin 127.0.0.1:8000/x/

.. code-block:: bash :class: dotted

HTTP/1.1 404 NOT FOUND Date: ..., ... .... ........ GMT Connection: keep-alive Server: h2o/2.2.4 content-length: 0

Compare and Swap (CAS)

A form of

opportunistic concurrency control 
_ is available through
ETags 
_.

When the client provides the Prefers: ETag header on a GET request we generate an ETag. A client can then use the

If-Match 
_ header with the ETag to perform a conditional update, (ie. a CAS operation). If the ETag has changed then the PUT operation will fail. CAS operations are great because there is no locking; if a CAS operation fails for one client that means it has succeeded for another, ie. there has been progress.

Imagine two clients trying to update the same key. Client 1 requests an ETag. The ETag is provided via the etag header.

.. code-block:: bash

echo 'SWEET DATA' | http -h --ignore-stdin PUT 127.0.0.1:8000/x/ > /dev/null http -h --ignore-stdin GET 127.0.0.1:8000/x/ Prefers:ETag > etag.txt cat etag.txt

.. code-block:: bash :class: dotted

HTTP/1.1 200 OK Date: ..., ... .... ........ GMT Connection: keep-alive Server: h2o/2.2.4 etag: ... transfer-encoding: chunked

If client 1 requests an ETag again, the same ETag is sent:

.. code-block:: bash

http -h --ignore-stdin GET 127.0.0.1:8000/x/ Prefers:ETag > etag2.txt cat etag2.txt diff <(grep etag etag.txt) <(grep etag etag2.txt)

.. code-block:: bash :class: dotted

HTTP/1.1 200 OK Date: ..., ... .... ........ GMT Connection: keep-alive Server: h2o/2.2.4 etag: ... transfer-encoding: chunked

Client 2 does a PUT on x. This will invalidate the ETag.

.. code-block:: bash

echo 'SURPRISE' | http -h PUT 127.0.0.1:8000/x/

.. code-block:: bash :class: dotted

HTTP/1.1 200 OK Date: ..., ... .... ........ GMT Connection: keep-alive Server: h2o/2.2.4 transfer-encoding: chunked

Client 1 uses a conditional PUT to update "x" using the If-Match tag. Because the ETag was invalidated, we don't commit, and respond with 412 Precondition Failed.

.. code-block:: bash

echo 'MY NEW VALUE BASED OFF OLD VALUE' | http -h PUT 127.0.0.1:8000/x/ If-Match:$(grep etag: etag.txt | sed -e 's/etag: //' | tr -d '\r\n')

.. code-block:: bash :class: dotted

HTTP/1.1 412 BAD ETAG Date: ..., ... .... ........ GMT Connection: keep-alive Server: h2o/2.2.4 content-length: 0

Once this happens we can retry the PUT after we do a new GET.

.. code-block:: bash

http -h GET 127.0.0.1:8000/x/ Prefers:ETag > etag3.txt cat etag3.txt

.. code-block:: bash :class: dotted

HTTP/1.1 200 OK Date: ..., ... .... ........ GMT Connection: keep-alive Server: h2o/2.2.4 etag: ... transfer-encoding: chunked

The PUT will succeed because the ETag is still valid.

.. code-block:: bash

echo 'NEW VALUE' | http -h PUT 127.0.0.1:8000/x/ If-Match:$(grep etag: etag3.txt | sed -e 's/etag: //' | tr -d '\r\n')

.. code-block:: bash :class: dotted

HTTP/1.1 200 OK Date: ..., ... .... ........ GMT Connection: keep-alive Server: h2o/2.2.4 transfer-encoding: chunked

However, if we use the ETag again it will fail.

.. code-block:: bash

echo 'NEW VALUE2' | http -h PUT 127.0.0.1:8000/x/ If-Match:$(grep etag: etag3.txt | sed -e 's/etag: //' | tr -d '\r\n')

.. code-block:: bash :class: dotted

HTTP/1.1 412 BAD ETAG Date: ..., ... .... ........ GMT Connection: keep-alive Server: h2o/2.2.4 content-length: 0

Notes about ETags:

  • On reboots, PearlDB loses all ETag information
  • On launch PearlDB generates a random ETag prefix
  • ETags are expected to have a short life (ie. < 1 day)

OPTIONS

You can check what HTTP methods are available to a resource using the OPTIONS method. This is useful as some systems like HAProxy use the OPTIONS method as a

healthcheck 
_.

.. code-block:: bash

http -h --ignore-stdin OPTIONS 127.0.0.1:8000/x/

.. code-block:: bash :class: dotted

HTTP/1.1 200 OK Date: ..., ... .... ........ GMT Connection: keep-alive Server: h2o/2.2.4 allow: HEAD,GET,PUT,DELETE,OPTIONS transfer-encoding: chunked

.. code-block:: bash

http -h --ignore-stdin OPTIONS 127.0.0.1:8000/

.. code-block:: bash :class: dotted

HTTP/1.1 200 OK Date: ..., ... .... ........ GMT Connection: keep-alive Server: h2o/2.2.4 allow: POST,OPTIONS transfer-encoding: chunked

Shutting down

.. code-block:: bash

cat pearl.pid | xargs kill -9 echo shutdown

.. code-block:: bash :class: dotted

shutdown

Building

.. code-block:: bash :class: ignore

sudo apt-get install git cmake automake libtool libssl-dev make libuv make libh2o make libck make

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