title: redis master does not see slave
body: <p>I have an issue with my redis sentinel setup, which has 4 nodes (1 master, 3 slaves). I've patched first slave node (docker version changed from 17.03.1-ce to 17.12.0-ce). My problem is that master does not anymore take  slave node1 to the members pool.</p>
  
  <p>Slave (node1) info (It recognized master node):</p>
  
  <pre><code>$ docker exec -it redis-sentinel redis-cli info replication
   # Replication
   role:slave
   master_host:&lt;master_ip&gt;
   master_port:6379
   master_link_status:down
  </code></pre>
  
  <p>Master info:</p>
  
  <pre><code>$ docker exec -it redis-sentinel redis-cli info replication
  # Replication
  role:master
  connected_slaves:2    
  slave0:ip=&lt;slave_2_ip&gt;,port=6379,state=online,offset=191580670534,lag=0   
  slave1:ip=&lt;slave_3_ip&gt;,port=6379,state=online,offset=191580666435,lag=0
  master_repl_offset:191580672343
  </code></pre>
  
  <p>Master must have 3 slaves. Master IP is correct on node1 (what was patched). node 2,3,4 docker versions are 17.03.1-ce. When I tested the same situation in development - all works fine. Can you suggest something, what I need to do, to enable replication between master and slave node1?</p>
  
  <p>After docker restart (@node1) I see something like that (msg="unknown container"):</p>
  
  <pre><code>Jan 31 08:16:12 node1 dockerd[17288]: time="2018-01-31T08:16:12.150892519+02:00" level=warning msg="unknown container" container=23e48b7846bd325ba5af772217085b60708660f5f5d8bb6fefd23094235ac01f module=libcontainerd namespace=plugins.moby
  Jan 31 08:16:12 node1 dockerd[17288]: time="2018-01-31T08:16:12.177513187+02:00" level=warning msg="unknown container" container=23e48b7846bd325ba5af772217085b60708660f5f5d8bb6fefd23094235ac01f module=libcontainerd namespace=plugins.moby
  </code></pre>
  
  <p>When I examine node4 master logs I see that node1 was converted to slave:</p>
  
  <pre><code>1:X 30 Jan 21:35:09.301 # +sdown sentinel 66f6a8950a72952ac7df18f6a653718445fad5db node1_slave 26379 @ sentinel-xx node4_master 6379
  1:X 30 Jan 21:35:10.276 # +sdown slave node1_slave:6379 node1_slave 6379 @ sentinel-xx node4_master 6379
  1:X 30 Jan 21:58:10.388 * +reboot slave node1_slave:6379 node1_slave 6379 @ sentinel-xx node4_master 6379
  1:X 30 Jan 21:58:10.473 # -sdown slave node1_slave:6379 node1_slave 6379 @ sentinel-xx node4_master 6379
  1:X 30 Jan 21:58:10.473 # -sdown sentinel 66f6a8950a72952ac7df18f6a653718445fad5db node1_slave 26379 @ sentinel-xx node4_master 6379
  1:X 30 Jan 21:58:20.436 * +convert-to-slave slave node1_slave:6379 node1_slave 6379 @ sentinel-xx node4_master 6379
  1:X 30 Jan 21:58:30.516 * +convert-to-slave slave node1_slave:6379 node1_slave 6379 @ sentinel-xx node4_master 6379
  1:X 30 Jan 21:58:40.529 * +convert-to-slave slave node1_slave:6379 node1_slave 6379 @ sentinel-xx node4_master 6379
  1:X 30 Jan 22:39:48.284 * +reboot slave node1_slave:6379 node1_slave 6379 @ sentinel-xx node4_master 6379
  1:X 30 Jan 22:39:58.391 * +convert-to-slave slave node1_slave:6379 node1_slave 6379 @ sentinel-xx node4_master 6379
  1:X 30 Jan 22:40:08.447 * +convert-to-slave slave node1_slave:6379 node1_slave 6379 @ sentinel-xx node4_master 6379
  </code></pre>
  
  <p>On the other hand redis-client logs are showing me that it cannot save DB on disk.</p>
  
  <pre><code>$ docker logs --follow redis-client
  1:M 31 Jan 07:47:09.451 * Slave node3_slave:6379 asks for synchronization
  1:M 31 Jan 07:47:09.451 * Full resync requested by slave node3_slave:6379
  1:M 31 Jan 07:47:09.451 * Starting BGSAVE for SYNC with target: disk
  1:M 31 Jan 07:47:09.452 # Can't save in background: fork: Out of memory
  1:M 31 Jan 07:47:09.452 # BGSAVE for replication failed
  1:M 31 Jan 07:47:24.628 * Slave node1_slave:6379 asks for synchronization
  1:M 31 Jan 07:47:24.628 * Full resync requested by slave node1_slave:6379
  1:M 31 Jan 07:47:24.628 * Starting BGSAVE for SYNC with target: disk
  1:M 31 Jan 07:47:24.628 # Can't save in background: fork: Out of memory
  1:M 31 Jan 07:47:24.628 # BGSAVE for replication failed
  1:M 31 Jan 07:48:10.560 * Slave node3_slave:6379 asks for synchronization
  1:M 31 Jan 07:48:10.560 * Full resync requested by slave node3_slave:6379
  1:M 31 Jan 07:48:10.560 * Starting BGSAVE for SYNC with target: disk
  </code></pre>