root / doc / design-2.0-master-daemon.rst @ bb083b25
History | View | Annotate | Download (7.1 kB)
1 |
Ganeti 2.0 Master daemon |
---|---|
2 |
======================== |
3 |
|
4 |
Objective |
5 |
--------- |
6 |
|
7 |
Many of the important features of Ganeti 2.0 — job queue, granular |
8 |
locking, external API, etc. — will be integrated via a master |
9 |
daemon. While not absolutely necessary, it is the best way to |
10 |
integrate all these components. |
11 |
|
12 |
Background |
13 |
---------- |
14 |
|
15 |
Currently there is no "master" daemon in Ganeti (1.2). Each command |
16 |
tries to acquire the so called *cmd* lock and when it succeeds, it |
17 |
takes complete ownership of the cluster configuration and state. The |
18 |
scheduled improvements to Ganeti require or can use a daemon that |
19 |
coordinates the activities/jobs scheduled/etc. |
20 |
|
21 |
Overview |
22 |
-------- |
23 |
|
24 |
The master daemon will be the central point of the cluster; command |
25 |
line tools and the external API will interact with the cluster via |
26 |
this daemon; it will be one coordinating the node daemons. |
27 |
|
28 |
This design doc is best read in the context of the accompanying design |
29 |
docs for Ganeti 2.0: Granular locking design and Job queue design. |
30 |
|
31 |
|
32 |
Detailed Design |
33 |
--------------- |
34 |
|
35 |
In Ganeti 2.0, we will have the following *entities*: |
36 |
|
37 |
- the master daemon (on master node) |
38 |
- the node daemon (all nodes) |
39 |
- the command line tools (master node) |
40 |
- the RAPI daemon (master node) |
41 |
|
42 |
Interaction paths are between: |
43 |
|
44 |
- (CLI tools/RAPI daemon) and the master daemon, via the so called *luxi* API |
45 |
- the master daemon and the node daemons, via the node RPC |
46 |
|
47 |
The protocol between the master daemon and the node daemons will be |
48 |
changed to HTTP(S), using a simple PUT/GET of JSON-encoded |
49 |
messages. This is done due to difficulties in working with the twisted |
50 |
protocols in a multithreaded environment, which we can overcome by |
51 |
using a simpler stack (see the caveats section). The protocol between |
52 |
the CLI/RAPI and the master daemon will be a custom one: on a UNIX |
53 |
socket on the master node, with rights restricted by filesystem |
54 |
permissions, the CLI/API will speak HTTP to the master daemon. |
55 |
|
56 |
The operations supported over this internal protocol will be encoded |
57 |
via a python library that will expose a simple API for its |
58 |
users. Internally, the protocol will simply encode all objects in JSON |
59 |
format and decode them on the receiver side. |
60 |
|
61 |
The LUXI protocol |
62 |
~~~~~~~~~~~~~~~~~ |
63 |
|
64 |
We will have two main classes of operations over the master daemon API: |
65 |
|
66 |
- cluster query functions |
67 |
- job related functions |
68 |
|
69 |
The cluster query functions are usually short-duration, and are the |
70 |
equivalent of the OP_QUERY_* opcodes in ganeti 1.2 (and they are |
71 |
internally implemented still with these opcodes). The clients are |
72 |
guaranteed to receive the response in a reasonable time via a timeout. |
73 |
|
74 |
The job-related functions will be: |
75 |
|
76 |
- submit job |
77 |
- query job (which could also be categorized in the query-functions) |
78 |
- archive job (see the job queue design doc) |
79 |
- wait for job change, which allows a client to wait without polling |
80 |
|
81 |
Daemon implementation |
82 |
~~~~~~~~~~~~~~~~~~~~~ |
83 |
|
84 |
The daemon will be based around a main I/O thread that will wait for |
85 |
new requests from the clients, and that does the setup/shutdown of the |
86 |
other thread (pools). |
87 |
|
88 |
|
89 |
There will two other classes of threads in the daemon: |
90 |
|
91 |
- job processing threads, part of a thread pool, and which are |
92 |
long-lived, started at daemon startup and terminated only at shutdown |
93 |
time |
94 |
- client I/O threads, which are the ones that talk the local protocol |
95 |
to the clients |
96 |
|
97 |
Master startup/failover |
98 |
~~~~~~~~~~~~~~~~~~~~~~~ |
99 |
|
100 |
In Ganeti 1.x there is no protection against failing over the master |
101 |
to a node with stale configuration. In effect, the responsibility of |
102 |
correct failovers falls on the admin. This is true both for the new |
103 |
master and for when an old, offline master startup. |
104 |
|
105 |
Since in 2.x we are extending the cluster state to cover the job queue |
106 |
and have a daemon that will execute by itself the job queue, we want |
107 |
to have more resilience for the master role. |
108 |
|
109 |
The following algorithm will happen whenever a node is ready to |
110 |
transition to the master role, either at startup time or at node |
111 |
failover: |
112 |
|
113 |
#. read the configuration file and parse the node list |
114 |
contained within |
115 |
|
116 |
#. query all the nodes and make sure we obtain an agreement via |
117 |
a quorum of at least half plus one nodes for the following: |
118 |
|
119 |
- we have the latest configuration and job list (as |
120 |
determined by the serial number on the configuration and |
121 |
highest job ID on the job queue) |
122 |
|
123 |
- there is not even a single node having a newer |
124 |
configuration file |
125 |
|
126 |
- if we are not failing over (but just starting), the |
127 |
quorum agrees that we are the designated master |
128 |
|
129 |
#. at this point, the node transitions to the master role |
130 |
|
131 |
#. for all the in-progress jobs, mark them as failed, with |
132 |
reason unknown or something similar (master failed, etc.) |
133 |
|
134 |
|
135 |
Logging |
136 |
~~~~~~~ |
137 |
|
138 |
The logging system will be switched completely to the logging module; |
139 |
currently it's logging-based, but exposes a different API, which is |
140 |
just overhead. As such, the code will be switched over to standard |
141 |
logging calls, and only the setup will be custom. |
142 |
|
143 |
With this change, we will remove the separate debug/info/error logs, |
144 |
and instead have always one logfile per daemon model: |
145 |
|
146 |
- master-daemon.log for the master daemon |
147 |
- node-daemon.log for the node daemon (this is the same as in 1.2) |
148 |
- rapi-daemon.log for the RAPI daemon logs |
149 |
- rapi-access.log, an additional log file for the RAPI that will be |
150 |
in the standard http log format for possible parsing by other tools |
151 |
|
152 |
Since the watcher will only submit jobs to the master for startup of |
153 |
the instances, its log file will contain less information than before, |
154 |
mainly that it will start the instance, but not the results. |
155 |
|
156 |
Caveats |
157 |
------- |
158 |
|
159 |
A discussed alternative is to keep the current individual processes |
160 |
touching the cluster configuration model. The reasons we have not |
161 |
chosen this approach is: |
162 |
|
163 |
- the speed of reading and unserializing the cluster state |
164 |
today is not small enough that we can ignore it; the addition of |
165 |
the job queue will make the startup cost even higher. While this |
166 |
runtime cost is low, it can be on the order of a few seconds on |
167 |
bigger clusters, which for very quick commands is comparable to |
168 |
the actual duration of the computation itself |
169 |
|
170 |
- individual commands would make it harder to implement a |
171 |
fire-and-forget job request, along the lines "start this |
172 |
instance but do not wait for it to finish"; it would require a |
173 |
model of backgrounding the operation and other things that are |
174 |
much better served by a daemon-based model |
175 |
|
176 |
Another area of discussion is moving away from Twisted in this new |
177 |
implementation. While Twisted hase its advantages, there are also many |
178 |
disatvantanges to using it: |
179 |
|
180 |
- first and foremost, it's not a library, but a framework; thus, if |
181 |
you use twisted, all the code needs to be 'twiste-ized'; we were able |
182 |
to keep the 1.x code clean by hacking around twisted in an |
183 |
unsupported, unrecommended way, and the only alternative would have |
184 |
been to make all the code be written for twisted |
185 |
- it has some weaknesses in working with multiple threads, since its base |
186 |
model is designed to replace thread usage by the deffered, so while it can |
187 |
use threads, it's not less flexible in doing so |
188 |
|
189 |
And, since we already have an http server library (for the RAPI), we |
190 |
can just reuse that for inter-node communication. |