NAME

Data::Chronicle - Chronicle storage system

DESCRIPTION

This package contains three modules (Reader, Writer, and Subscriber) which can be used to store and retrieve information on an efficient storage with below properties:

Timeliness

It is assumed that data to be stored are time-based meaning they change over time and the latest version is most important for us.

Efficient

The module uses Redis cache to provide efficient data storage and retrieval.

Persistent

In addition to caching every incoming data, it is also stored in PostgreSQL for future retrieval.

Transparent

This modules hides all the details about caching, database structure and ... from developer. He only needs to call a method to save data and another method to retrieve it. All the underlying complexities are handled by the module.

Note that you will need to pass `cache_writer`, `cache_reader` and `dbic` to the `Data::Chronicle::Reader/Writer` modules. These three arguments, provide access to your Redis and PostgreSQL which will be used by Chronicle modules.

`cache_writer` and `cache_reader` should be to be able to get/set given data under given key (both of type string). `dbic` should be capable to store and retrieve data with `category`,`name` in addition to the timestamp of data insertion. So it should be able to retrieve data for a specific timestamp, category and name. Category, name and data are all string. This can easily be achieved by defining a table in you database containing these columns: `timestamp, category, name, value`.

METHODS

There are four important methods this module provides:

"set" in Data::Chronicle::Writer

Given a category, name and value stores the JSONified value in Redis and PostgreSQL database under "category::name" group and also stores current system time as the timestamp for the data (Which can be used for future retrieval if we want to get data as of a specific time). Note that the value MUST be either hash-ref or array-ref.

    $writer->set("category1", "name1", "value1");
    $writer->set("category1", "name2", "value2", Date::Utility->new("2016-08-01 00:06:00"));

"mset" in Data::Chronicle::Writer

Given multiple categories, names and values atomically performs the set operation on each corresponding category, name, value set.

    $writer->mset([["category1", "name1", $value1], ["category2, "name2", $value2], ...]);

"get" in Data::Chronicle::Reader

Given a category and name returns the latest version of the data according to current Redis cache

    my $value1 = $reader->get("category1, "name1"); #value1

"mget" in Data::Chronicle::Reader

Given multiple categories and name atomically performs the get operation on each corresponding category, name set.

    my @values = $reader->mget([["category1", "name1"], ["category2", "name2"], ...])

"get_for" in Data::Chronicle::Reader

Given a category, name and timestamp returns version of data under "category::name" as of the given date (using a DB lookup).

    my $some_old_data = $reader->get_for("category1", "name2", Date::Utility->new("2016-08-01 00:06:00"));

"get_for_period" in Data::Chronicle::Reader

Given a category, name, start_timestamp and end_timestamp returns an array-ref containing all data stored between given period for the given "category::name" (using a DB lookup).

    my $arrayref = $reader->get_for_period("category1", "name2", Date::Utility->new("2015-08-01 00:06:00"), Date::Utility->new("2015-08-01 00:06:00"));

"get_history" in Data::Chronicle::Reader

Given a category, name, and revision returns version of the data the specified number of revisions in the past. If revision 0 is chosen, the latest version of the data will be returned. If revision 1 is chosen, the previous version of the data will be returned.

    my $some_old_data = $reader->get_for("category1", "name2", 2);

"subscribe" in Data::Chronicle::Subscriber

Given a category, name, and callback assigns the callback to be called when a new value is set for the specified category and name (if the writer has publish_on_set enabled).

    $subscriber->subscribe("category1", "name2", sub { print 'Hello World' });

"unsubscribe" in Data::Chronicle::Subscriber

Given a category, name, clears the callbacks associated with the specified category and name.

    $subscriber->unsubscribe("category1", "name2");

EXAMPLES

    my $d = get_some_log_data();

    my $chronicle_w = Data::Chronicle::Writer->new(
        cache_writer => $writer,
        dbic         => $dbic);

    my $chronicle_r = Data::Chronicle::Reader->new(
        cache_reader => $reader,
        dbic         => $dbic);


    #store data into Chronicle - each time we call `set` it will also store
    #a copy of the data for historical data retrieval
    $chronicle_w->set("log_files", "syslog", $d);

    #retrieve latest data stored for syslog under log_files category
    my $dt = $chronicle_r->get("log_files", "syslog");

    #find historical data for `syslog` at given point in time
    my $some_old_data = $chronicle_r->get_for("log_files", "syslog", $epoch1);