87 lines
3.4 KiB
C#
87 lines
3.4 KiB
C#
using System;
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using System.Threading;
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using System.Threading.Tasks;
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using MediaBrowser.Controller.ScheduledTasks;
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using Microsoft.Extensions.Logging;
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using Microsoft.Extensions.Options;
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using StackExchange.Redis;
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namespace Emby.Server.Implementations.ScheduledTasks;
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/// <summary>
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/// A Redis-backed <see cref="IScanLeaderLease"/> that elects a single scan-leader instance using a
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/// TTL lease on a shared key. The lease value is this instance's pod identity; a leader that keeps
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/// renewing retains the lease, and any instance can claim it once the previous leader's lease expires.
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/// </summary>
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/// <remarks>
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/// This fails open on an unreachable Redis while quick connect's startup read fails closed on the same
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/// connection. They do not compete: the startup read decides whether the instance runs at all, and this
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/// only decides what a running instance does about a store that went away afterwards.
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/// </remarks>
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public sealed class RedisScanLeaderLease : IScanLeaderLease
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{
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private const string LeaderKey = "jellyfin:scanleader";
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/// <summary>
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/// Lua script for atomic acquire-or-renew: if the key is unset (missing or already expired) it is
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/// set to this pod for the lease duration and 1 is returned; if it already holds this pod the TTL is
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/// extended and 1 is returned; otherwise another pod owns a live lease and 0 is returned.
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/// </summary>
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private const string AcquireOrRenewScript = @"
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local current = redis.call('GET', KEYS[1])
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if not current then
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redis.call('SET', KEYS[1], ARGV[1], 'PX', ARGV[2])
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return 1
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elseif current == ARGV[1] then
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redis.call('PEXPIRE', KEYS[1], ARGV[2])
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return 1
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else
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return 0
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end";
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private readonly IDatabase _db;
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private readonly ScanLeaderOptions _options;
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private readonly ILogger<RedisScanLeaderLease> _logger;
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private readonly string _podId;
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/// <summary>
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/// Initializes a new instance of the <see cref="RedisScanLeaderLease"/> class.
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/// </summary>
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/// <param name="redis">The Redis connection multiplexer.</param>
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/// <param name="options">The scan-leader configuration options.</param>
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/// <param name="logger">The logger.</param>
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public RedisScanLeaderLease(
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IConnectionMultiplexer redis,
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IOptions<ScanLeaderOptions> options,
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ILogger<RedisScanLeaderLease> logger)
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{
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_db = redis.GetDatabase();
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_options = options.Value;
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_logger = logger;
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_podId = Environment.GetEnvironmentVariable("JELLYFIN_INSTANCE_ID") ?? Environment.MachineName;
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}
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/// <inheritdoc />
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public async Task<bool> TryAcquireOrRenewAsync(CancellationToken cancellationToken = default)
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{
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var leaseDurationMs = (long)_options.LeaseDurationSeconds * 1000;
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try
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{
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var result = (long?)await _db.ScriptEvaluateAsync(
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AcquireOrRenewScript,
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keys: new RedisKey[] { LeaderKey },
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values: new RedisValue[] { _podId, leaseDurationMs }).ConfigureAwait(false);
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return result == 1;
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}
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catch (Exception ex)
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{
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// Fail-safe: if Redis is unreachable, treat this instance as the leader so scheduled scans
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// keep running. Every instance scanning is preferable to no instance scanning.
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_logger.LogWarning(ex, "Scan-leader lease evaluation failed; treating {PodId} as leader.", _podId);
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return true;
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}
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}
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}
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