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Sun, 15 Jan 2017

Perl 6 By Example: Stateful Silent Cron


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This blog post is part of my ongoing project to write a book about Perl 6.

If you're interested, please sign up for the mailing list at the bottom of the article, or here. It will be low volume (less than an email per month, on average).


In the last two installments we've looked at silent-cron, a wrapper around external programs that silences them in case their exit status is zero. But to make it really practical, it should also silence occasional failures.

External APIs fail, networks become congested, and other things happen that prevent a job from succeeding, so some kind of retry mechanism is desirable. In case of a cron job, cron already takes care of retrying a job on a regular basis, so silent-cron should just suppress occasional errors. On the other hand, if a job fails consistently, this is usually something that an operator or developer should look into, so it's a problem worth reporting.

To implement this functionality, silent-cron needs to store persistent state between separate runs. It needs to record the results from the current run and then analyze if the failure history qualifies as "occasional".

Persistent Storage

The storage backend needs to write and retrieve structured data, and protect concurrent access to the state file with locking. A good library for such a storage backend is SQLite, a zero-maintenance SQL engine that's available as a C library. It's public domain software and in use in most major browsers, operating systems and even some airliners.

Perl 6 gives you access to SQLite's functionality through DBIish, a generic database interface with backend drivers for SQLite, MySQL, PostgreSQL and Oracle DB. To use it, first make sure that SQLite3 is installed, including its header files. On a Debian-based Linux system, for example, you can achieve this with apt-get install libsqlite3-dev. If you are using the Rakudo Star distribution, DBIish is already available. If not, you can use one of the module installers to retrieve and install it: panda install DBIish or zef install DBIish.

To use the DBIish's SQLite backend, you first have to create a database handle by selecting the backend and supplying connection information:

use DBIish;
my $dbh = DBIish.connect('SQLite', :database('database-file.sqlite3'));

Connecting to a database file that does not yet exist creates that file.

One-off SQL statements can be executed directly on the database handle:

$dbh.do('INSERT INTO player (name) VALUES ?', 'John');

The ? in the SQL is a placeholder that is passed out-of-band as a separate argument to the do method, which avoids potential errors such as SQL injection vulnerabilities.

Queries tend to work by first preparing a statement which returns a statement handle. You can execute a statement once or multiple times, and retrieve result rows after each execute call:

my $sth = $dbh.prepare('SELECT id FROM player WHERE name = ?');

my %ids;
for <John Jack> -> $name {
    $sth.execute($name);
    %ids{ $name } = $sth.row[0];
}
$sth.finish;

Developing the Storage Backend

We shouldn't just stuff all the storage handling code into sub MAIN, we should instead carefully consider the creation of a useful API for the storage backend. At first, we need only two pieces of functionality: insert the result of a job execution; and retrieve the most recent results.

Since silent-cron can be used to guard multiple cron jobs on the same machine, we might need something to distinguish the different jobs so that one of them succeeding doesn't prevent error reporting for one that is constantly failing. For that we introduce a job name, which can default to the command (including arguments) being executed but which can be set explicitly on the command line.

The API for the storage backend could look something like this:

my $repo = ExecutionResultRepository.new(
    jobname   => 'refresh cache',
    statefile => 'silent-cron.sqlite3',
);
$repo.insert($result);
my @last-results = $repo.tail(5);

This API isn't specific to the SQLite backend at all; a storage backend that works with plain text files could have the exact same API.

Let's implement this API. First we need the class and the two attributes that should be obvious from the usage example above:

class ExecutionResultRepository {
    has $.jobname   is required;
    has $.statefile is required;
    # ... more code

To implement the insert method, we need to connect to the database and create the relevant table if it doesn't exist yet.

has $!db;
method !db() {
    return $!db if $!db;
    $!db = DBIish.connect('SQLite', :database($.statefile));
    self!create-schema();
    return $!db;
}

This code uses a private attribute $!db to cache the database handle and a private method !db to create the handle if it doesn't exist yet.

Private methods are declared like ordinary methods, except that the name starts with an exclamation mark. To call one, substitute the method call dot for the exclamation mark, in other words, use self!db() instead of self.db().

The !db method also calls the next private method, !create-schema, which creates the storage table and some indexes:

method !create-schema() {
    $!db.do(qq:to/SCHEMA/);
        CREATE TABLE IF NOT EXISTS $table (
            id          INTEGER PRIMARY KEY,
            jobname     VARCHAR NOT NULL,
            exitcode    INTEGER NOT NULL,
            timed_out   INTEGER NOT NULL,
            output      VARCHAR NOT NULL,
            executed    TIMESTAMP NOT NULL DEFAULT (DATETIME('NOW'))
        );
    SCHEMA
    $!db.do(qq:to/INDEX/);
        CREATE INDEX IF NOT EXISTS {$table}_jobname_exitcode ON $table ( jobname, exitcode );
    INDEX
    $!db.do(qq:to/INDEX/);
        CREATE INDEX IF NOT EXISTS {$table}_jobname_executed ON $table ( jobname, executed );
    INDEX
}

Multi-line string literals are best written with the heredoc syntax. qq:to/DELIMITER/ tells Perl 6 to finish parsing the current statement so that you can still close the method call parenthesis and add the statement-ending semicolon. The next line starts the string literal, which goes on until Perl 6 finds the delimiter on a line on its own. Leading whitespace is stripped from each line of the string literal by as much as the closing delimiter is indented.

For example

print q:to/EOS/;
    Not indented
        Indented four spaces
    EOS

Produces the output

Not indented
    Indented four spaces

Now that we have a working database connection and know that the database table exists, inserting a new record becomes easy:

method insert(ExecutionResult $r) {
    self!db.do(qq:to/INSERT/, $.jobname, $r.exitcode, $r.timed-out, $r.output);
        INSERT INTO $table (jobname, exitcode, timed_out, output)
        VALUES(?, ?, ?, ?)
    INSERT
}

Selecting the most recent records is a bit more work, partially because we need to convert the table rows into objects:

method tail(Int $count) {
    my $sth = self!db.prepare(qq:to/SELECT/);
        SELECT exitcode, timed_out, output
          FROM $table
         WHERE jobname = ?
      ORDER BY executed DESC
         LIMIT $count
    SELECT
    $sth.execute($.jobname);
    $sth.allrows(:array-of-hash).map: -> %h {
        ExecutionResult.new(
            exitcode  => %h<exitcode>,
            timed-out => ?%h<timed_out>,
            output    => %h<output>,
        );
    }
}

The last statement in the tail method deserves a bit of extra attention. $sth.allrows(:array-of-hash) produces the database rows as a list of hashes. This list is lazy, that is, it's generated on-demand. Lazy lists are a very convenient feature because they allow you to use iterators and lists with the same API. For example when reading lines from a file, you can write for $handle.lines -> $line { ... }, and the lines method doesn't have to load the whole file into memory; instead it can read a line whenever it is accessed.

$sth.allrows(...) is lazy, and so is the .map call that comes after it. map transforms a list one element at a time by calling the code object that's passed to it. And that is done lazily as well. So SQLite only retrieves rows from the database file when elements of the resulting list are actually accessed.

Using the Storage Backend

With the storage API in place, it's time to use it:

multi sub MAIN(*@cmd, :$timeout, :$jobname is copy,
               :$statefile='silent-cron.sqlite3', Int :$tries = 3) {
    $jobname //= @cmd.Str;
    my $result = run-with-timeout(@cmd, :$timeout);
    my $repo = ExecutionResultRepository.new(:$jobname, :$statefile);
    $repo.insert($result);

    my @runs = $repo.tail($tries);

    unless $result.is-success or @runs.grep({.is-success}) {
        say "The last @runs.elems() runs of @cmd[] all failed, the last execution ",
            $result.timed-out ?? "ran into a timeout"
                              !! "exited with code $result.exitcode()";

        print "Output:\n", $result.output if $result.output;
    }
    exit $result.exitcode // 2;
}

Now a job that succeeds a few times, and then fails up to two times in a row doesn't produce any error output, and only the third failed execution in a row produces output. You can override that on the command line with --tries=5.

Summary

We've discussed DBIish, a database API with pluggable backend, and explored using it with SQLite to store persistent data. In the process we also came across lazy lists and a new form of string literals called heredocs.

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