# Consistency Level Console Demo
In this demo, we’ll bring up 3 nodes and demonstrate how writes and reads look, with tracing enabled in a cluster where our [Replication Factor (RF)](https://docs.scylladb.com/manual/master/reference/glossary.md#term-Replication-Factor-RF) is set to **3**. We’ll change the [Consistency Level (CL)](https://docs.scylladb.com/manual/master/reference/glossary.md#term-Consistency-Level-CL) between operations to show how messages are passed between nodes, and finally take down a few nodes to show failure conditions in a ScyllaDB cluster.
You can also learn more in the [High Availability lesson](https://university.scylladb.com/courses/scylla-essentials-overview/lessons/high-availability/) on ScyllaDB University.
Note: We use [asciinema](https://asciinema.org) to generate the console casts used in this demo. These asciicasts are more readable than embedded video, and allow you to copy the text or commands directly from the console to your clipboard. We suggest viewing console casts in **fullscreen** to see the properly formatted output.
## Step one: bring up the cluster
First, we’ll bring up a 3-node docker cluster. You’ll see all the nodes eventually read `UN` for status. `U` means **up**, and `N` means **normal**. Read more about [Nodetool Status](https://docs.scylladb.com/manual/master/operating-scylla/nodetool-commands/status.md).
Once all nodes are up, we can run our CQL shell.
We’ve run `nodetool status` after we’ve waited for all nodes to come up:
```console
Datacenter: datacenter1
=======================
Status=Up/Down
|/ State=Normal/Leaving/Joining/Moving
-- Address Load Tokens Owns (effective) Host ID Rack
UN 172.17.0.3 127.42 KB 256 65.8% 66a3bfe7-3e90-4859-a394-becc00af1cf2 rack1
UN 172.17.0.2 110.58 KB 256 65.5% f66639bf-3259-47f5-8487-602fcb082624 rack1
UN 172.17.0.4 118.38 KB 256 68.6% d978ff65-0331-4f27-a129-9be2d7169ad6 rack1
```
## Step two: take a single node down, check different consistency levels
So, what happens when we take a single node down? Here, we’ll take a single node down, check status with `nodetool`, attempt a write with **ALL** and **QUORUM** consistency levels.
Note that we’ve turned tracing off. We can observe that our **CL=ALL** writes will always fail because one of the nodes is down. **CL=QUORUM** operations *could* fail, depending on whether a required partition replica falls on a node that is up or down.
## Step three: take 2 nodes down, read and write with different consistency levels
Let’s take a second node down and attempt to read and write with QUORUM and ONE consistency levels.
With 2 nodes down on our 3 node cluster and **RF=3**, a **CL=QUORUM** will always fail as at least one of the required nodes will always be down. One the other hand, a **CL=ONE** will succeed, as it only requires one up node.
## Step four: testing different consistency levels with tracing on
Finally, we’ll turn on tracing, and with all our nodes up, write to the table with **CL=ALL** and read from it with **CL=QUORUM**.
We’ve just written a record with our consistency level set to ALL.
```console
cqlsh:mykeyspace> CONSISTENCY ALL
```
Consistency level set to ALL.
```console
cqlsh:mykeyspace> CREATE TABLE users(user_id int PRIMARY KEY, fname text, lname text);
```
```console
cqlsh:mykeyspace> TRACING ON
```
Now Tracing is enabled
```console
cqlsh:mykeyspace> insert into users(user_id, lname, fname) values (1, 'tzach', 'livyatan');
```
Here’s the output:
```console
Tracing session: 84deb7a0-9d81-11e7-b770-000000000002
activity | timestamp | source | source_elapsed
-------------------------------------------------------------------------+----------------------------+------------+---------------
Execute CQL3 query | 2017-09-19 21:28:46.362000 | 172.17.0.4 | 0
Parsing a statement [shard 0] | 2017-09-19 21:28:46.362223 | 172.17.0.4 | 14
Processing a statement [shard 0] | 2017-09-19 21:28:46.362604 | 172.17.0.4 | 273
Creating write handler for token: -4069959284402364209 | 2017-09-19 21:28:46.362962 | 172.17.0.4 | 755
natural: {172.17.0.3, 172.17.0.4, 172.17.0.2} pending: {} [shard 0] | | |
Creating write handler with live: {172.17.0.2, 172.17.0.3, 172.17.0.4} | 2017-09-19 21:28:46.362992 | 172.17.0.4 | 784
dead: {} [shard 0] | | |
Sending a mutation to /172.17.0.2 [shard 0] | 2017-09-19 21:28:46.363171 | 172.17.0.4 | 959
Sending a mutation to /172.17.0.3 [shard 0] | 2017-09-19 21:28:46.363298 | 172.17.0.4 | 1090
Executing a mutation locally [shard 0] | 2017-09-19 21:28:46.363316 | 172.17.0.4 | 1109
Message received from /172.17.0.4 [shard 1] | 2017-09-19 21:28:46.363942 | 172.17.0.3 | 16
Sending mutation_done to /172.17.0.4 [shard 1] | 2017-09-19 21:28:46.364225 | 172.17.0.3 | 307
Message received from /172.17.0.4 [shard 1] | 2017-09-19 21:28:46.365395 | 172.17.0.2 | 93
Sending mutation_done to /172.17.0.4 [shard 1] | 2017-09-19 21:28:46.365561 | 172.17.0.2 | 206
Mutation handling is done [shard 1] | 2017-09-19 21:28:46.366538 | 172.17.0.2 | 1234
Got a response from /172.17.0.2 [shard 0] | 2017-09-19 21:28:46.368064 | 172.17.0.4 | 5858
Got a response from /172.17.0.4 [shard 0] | 2017-09-19 21:28:46.369589 | 172.17.0.4 | 7384
Mutation handling is done [shard 1] | 2017-09-19 21:28:46.370843 | 172.17.0.3 | 6928
Got a response from /172.17.0.3 [shard 0] | 2017-09-19 21:28:46.372486 | 172.17.0.4 | 9964
Mutation successfully completed [shard 0] | 2017-09-19 21:28:46.372519 | 172.17.0.4 | 10314
Done processing - preparing a result [shard 0] | 2017-09-19 21:28:46.372548 | 172.17.0.4 | 10341
Request complete | 2017-09-19 21:28:46.372357 | 172.17.0.4 | 10357
```
With tracing on, we have verbose output. Also, with all nodes up, we should have no errors experiencing a **CL=ALL** write. (Later, we will try different consistency levels with 1 and 2 nodes down.) With **RF=3**, we see all three nodes involved in the interaction. Once the co-ordinator (`172.17.0.4`) has received responses from the two other replicas - `172.17.0.2` and `172.17.0.3` - as well as itself- the operation is complete.
So what happens when we write under **QUORUM** instead of **ALL**? **QUORUM** means that we only require responses from (Replication Factor/2) + 1 nodes. Our replication factor is 3, therefore only a majority of 2 nodes must provide an acknowledgement for the eventually-consistent write to be considered successful. Below, we observe that our coordinator, after sending two other mutations to replicas `172.17.0.2` and `172.17.0.3`, executes a mutation locally. However, our coordinator only requires a write acknowledgement from itself and `172.17.0.3` before `Mutation successfully completed` is returned. Note that the response from `172.17.0.2` actually comes later.
```console
cqlsh:mykeyspace> CONSISTENCY QUORUM
```
Consistency level set to QUORUM.
```console
cqlsh:mykeyspace> insert into users (user_id, fname, lname) values (2, 'john', 'hammink');
```
Output:
```console
Tracing session: ccb9d1d0-98d7-11e7-b971-000000000003
activity | timestamp | source | source_elapsed
-------------------------------------------------------------------------+----------------------------+------------+---------------
Execute CQL3 query | 2017-09-13 23:03:47.821000 | 172.17.0.4 | 0
Parsing a statement [shard 3] | 2017-09-13 23:03:47.821879 | 172.17.0.4 | 1
Processing a statement [shard 3] | 2017-09-13 23:03:47.822050 | 172.17.0.4 | 172
Creating write handler for token: -3248873570005575792 | 2017-09-13 23:03:47.822160 | 172.17.0.4 | 283
natural: {172.17.0.3, 172.17.0.4, 172.17.0.2} pending: {} [shard 3] | | |
Creating write handler with live: {172.17.0.2, 172.17.0.3, 172.17.0.4} | 2017-09-13 23:03:47.822171 | 172.17.0.4 | 293
dead: {} [shard 3] | | |
Sending a mutation to /172.17.0.2 [shard 3] | 2017-09-13 23:03:47.822192 | 172.17.0.4 | 314
Sending a mutation to /172.17.0.3 [shard 3] | 2017-09-13 23:03:47.822209 | 172.17.0.4 | 331
Executing a mutation locally [shard 3] | 2017-09-13 23:03:47.822214 | 172.17.0.4 | 336
Message received from /172.17.0.4 [shard 1] | 2017-09-13 23:03:47.822564 | 172.17.0.2 | 7
Message received from /172.17.0.4 [shard 1] | 2017-09-13 23:03:47.822851 | 172.17.0.3 | 5
Sending mutation_done to /172.17.0.4 [shard 1] | 2017-09-13 23:03:47.823009 | 172.17.0.2 | 452
Mutation handling is done [shard 1] | 2017-09-13 23:03:47.823067 | 172.17.0.2 | 510
Sending mutation_done to /172.17.0.4 [shard 1] | 2017-09-13 23:03:47.823734 | 172.17.0.3 | 887
Mutation handling is done [shard 1] | 2017-09-13 23:03:47.823744 | 172.17.0.3 | 898
Got a response from /172.17.0.4 [shard 3] | 2017-09-13 23:03:47.823984 | 172.17.0.4 | 2106
Got a response from /172.17.0.3 [shard 3] | 2017-09-13 23:03:47.824998 | 172.17.0.4 | 3120
Mutation successfully completed [shard 3] | 2017-09-13 23:03:47.825003 | 172.17.0.4 | 3125
Done processing - preparing a result [shard 3] | 2017-09-13 23:03:47.825054 | 172.17.0.4 | 3176
Got a response from /172.17.0.2 [shard 3] | 2017-09-13 23:03:47.825696 | 172.17.0.4 | 3817
Request complete | 2017-09-13 23:03:47.824182 | 172.17.0.4 | 3182
```
We can observe that reading under **QUORUM** consistency works similarly. In our example below, our coordinator `172.17.0.4` waits for only `read_data` from `172.17.0.2` (as well as itself earlier: `read_data: querying locally`) before the read is considered successful. Note that node `172.17.0.3` similarly handles `read_digest` as `172.17.0.2` handles `read_data`.
```console
cqlsh:mykeyspace> CONSISTENCY QUORUM
```
Consistency level set to QUORUM.
```console
cqlsh:mykeyspace> select * from users where user_id = 1;
```
Output:
```console
user_id | fname | lname
---------+-------+----------
1 | tzach | livyatan
(1 rows)
Tracing session: b11084e0-9e33-11e7-9f6b-000000000003
activity | timestamp | source | source_elapsed
-------------------------------------------------------------------------+----------------------------+------------+---------------
Execute CQL3 query | 2017-09-20 18:44:10.926000 | 172.17.0.4 | 0
Parsing a statement [shard 3] | 2017-09-20 18:44:10.926442 | 172.17.0.4 | 1
Processing a statement [shard 3] | 2017-09-20 18:44:10.926538 | 172.17.0.4 | 97
Creating read executor for token -4069959284402364209 | 2017-09-20 18:44:10.926606 | 172.17.0.4 | 165
with all: {172.17.0.4, 172.17.0.2, 172.17.0.3} | | |
targets: {172.17.0.4, 172.17.0.2, 172.17.0.3} | | |
repair decision: DC_LOCAL [shard 3] | | |
read_digest: sending a message to /172.17.0.3 [shard 3] | 2017-09-20 18:44:10.926634 | 172.17.0.4 | 193
read_data: querying locally [shard 3] | 2017-09-20 18:44:10.926664 | 172.17.0.4 | 223
read_data: sending a message to /172.17.0.2 [shard 3] | 2017-09-20 18:44:10.926711 | 172.17.0.4 | 270
read_digest: message received from /172.17.0.4 [shard 3] | 2017-09-20 18:44:10.926956 | 172.17.0.3 | 6
read_data: message received from /172.17.0.4 [shard 3] | 2017-09-20 18:44:10.927310 | 172.17.0.2 | 7
read_digest handling is done, sending a response to /172.17.0.4 [shard 3]| 2017-09-20 18:44:10.927454 | 172.17.0.3 | 505
read_data handling is done, sending a response to /172.17.0.4 [shard 3] | 2017-09-20 18:44:10.928248 | 172.17.0.2 | 946
read_digest: got response from /172.17.0.3 [shard 3] | 2017-09-20 18:44:10.929873 | 172.17.0.4 | 3432
read_data: got response from /172.17.0.2 [shard 3] | 2017-09-20 18:44:10.929880 | 172.17.0.4 | 3439
Done processing - preparing a result [shard 3] | 2017-09-20 18:44:10.929934 | 172.17.0.4 | 3493
Request complete | 2017-09-20 18:44:10.929503 | 172.17.0.4 | 3503
```