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1 | ffedf64d | Michele Tartara | ========================== |
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2 | ffedf64d | Michele Tartara | Ganeti daemons refactoring |
3 | ffedf64d | Michele Tartara | ========================== |
4 | ffedf64d | Michele Tartara | |
5 | ffedf64d | Michele Tartara | .. contents:: :depth: 2 |
6 | ffedf64d | Michele Tartara | |
7 | ffedf64d | Michele Tartara | This is a design document detailing the plan for refactoring the internal |
8 | ffedf64d | Michele Tartara | structure of Ganeti, and particularly the set of daemons it is divided into. |
9 | ffedf64d | Michele Tartara | |
10 | ffedf64d | Michele Tartara | |
11 | ffedf64d | Michele Tartara | Current state and shortcomings |
12 | ffedf64d | Michele Tartara | ============================== |
13 | ffedf64d | Michele Tartara | |
14 | ffedf64d | Michele Tartara | Ganeti is comprised of a growing number of daemons, each dealing with part of |
15 | ffedf64d | Michele Tartara | the tasks the cluster has to face, and communicating with the other daemons |
16 | ffedf64d | Michele Tartara | using a variety of protocols. |
17 | ffedf64d | Michele Tartara | |
18 | ffedf64d | Michele Tartara | Specifically, as of Ganeti 2.8, the situation is as follows: |
19 | ffedf64d | Michele Tartara | |
20 | ffedf64d | Michele Tartara | ``Master daemon (MasterD)`` |
21 | ffedf64d | Michele Tartara | It is responsible for managing the entire cluster, and it's written in Python. |
22 | ffedf64d | Michele Tartara | It is executed on a single node (the master node). It receives the commands |
23 | ffedf64d | Michele Tartara | given by the cluster administrator (through the remote API daemon or the |
24 | ffedf64d | Michele Tartara | command line tools) over the LUXI protocol. The master daemon is responsible |
25 | ffedf64d | Michele Tartara | for creating and managing the jobs that will execute such commands, and for |
26 | ffedf64d | Michele Tartara | managing the locks that ensure the cluster will not incur in race conditions. |
27 | ffedf64d | Michele Tartara | |
28 | ffedf64d | Michele Tartara | Each job is managed by a separate Python thread, that interacts with the node |
29 | ffedf64d | Michele Tartara | daemons via RPC calls. |
30 | ffedf64d | Michele Tartara | |
31 | ffedf64d | Michele Tartara | The master daemon is also responsible for managing the configuration of the |
32 | ffedf64d | Michele Tartara | cluster, changing it when required by some job. It is also responsible for |
33 | ffedf64d | Michele Tartara | copying the configuration to the other master candidates after updating it. |
34 | ffedf64d | Michele Tartara | |
35 | ffedf64d | Michele Tartara | ``RAPI daemon (RapiD)`` |
36 | ffedf64d | Michele Tartara | It is written in Python and runs on the master node only. It waits for |
37 | ffedf64d | Michele Tartara | requests issued remotely through the remote API protocol. Then, it forwards |
38 | ffedf64d | Michele Tartara | them, using the LUXI protocol, to the master daemon (if they are commands) or |
39 | ffedf64d | Michele Tartara | to the query daemon if they are queries about the configuration (including |
40 | ffedf64d | Michele Tartara | live status) of the cluster. |
41 | ffedf64d | Michele Tartara | |
42 | ffedf64d | Michele Tartara | ``Node daemon (NodeD)`` |
43 | ffedf64d | Michele Tartara | It is written in Python. It runs on all the nodes. It is responsible for |
44 | ffedf64d | Michele Tartara | receiving the master requests over RPC and execute them, using the appropriate |
45 | ffedf64d | Michele Tartara | backend (hypervisors, DRBD, LVM, etc.). It also receives requests over RPC for |
46 | ffedf64d | Michele Tartara | the execution of queries gathering live data on behalf of the query daemon. |
47 | ffedf64d | Michele Tartara | |
48 | ffedf64d | Michele Tartara | ``Configuration daemon (ConfD)`` |
49 | ffedf64d | Michele Tartara | It is written in Haskell. It runs on all the master candidates. Since the |
50 | ffedf64d | Michele Tartara | configuration is replicated only on the master node, this daemon exists in |
51 | ffedf64d | Michele Tartara | order to provide information about the configuration to nodes needing them. |
52 | ffedf64d | Michele Tartara | The requests are done through ConfD's own protocol, HMAC signed, |
53 | ffedf64d | Michele Tartara | implemented over UDP, and meant to be used by parallely querying all the |
54 | ffedf64d | Michele Tartara | master candidates (or a subset thereof) and getting the most up to date |
55 | ffedf64d | Michele Tartara | answer. This is meant as a way to provide a robust service even in case master |
56 | ffedf64d | Michele Tartara | is temporarily unavailable. |
57 | ffedf64d | Michele Tartara | |
58 | ffedf64d | Michele Tartara | ``Query daemon (QueryD)`` |
59 | ffedf64d | Michele Tartara | It is written in Haskell. It runs on all the master candidates. It replies |
60 | ffedf64d | Michele Tartara | to Luxi queries about the current status of the system, including live data it |
61 | ffedf64d | Michele Tartara | obtains by querying the node daemons through RPCs. |
62 | ffedf64d | Michele Tartara | |
63 | ffedf64d | Michele Tartara | ``Monitoring daemon (MonD)`` |
64 | ffedf64d | Michele Tartara | It is written in Haskell. It runs on all nodes, including the ones that are |
65 | ffedf64d | Michele Tartara | not vm-capable. It is meant to provide information on the status of the |
66 | ffedf64d | Michele Tartara | system. Such information is related only to the specific node the daemon is |
67 | ffedf64d | Michele Tartara | running on, and it is provided as JSON encoded data over HTTP, to be easily |
68 | ffedf64d | Michele Tartara | readable by external tools. |
69 | ffedf64d | Michele Tartara | The monitoring daemon communicates with ConfD to get information about the |
70 | ffedf64d | Michele Tartara | configuration of the cluster. The choice of communicating with ConfD instead |
71 | ffedf64d | Michele Tartara | of MasterD allows it to obtain configuration information even when the cluster |
72 | ffedf64d | Michele Tartara | is heavily degraded (e.g.: when master and some, but not all, of the master |
73 | ffedf64d | Michele Tartara | candidates are unreachable). |
74 | ffedf64d | Michele Tartara | |
75 | ffedf64d | Michele Tartara | The current structure of the Ganeti daemons is inefficient because there are |
76 | ffedf64d | Michele Tartara | many different protocols involved, and each daemon needs to be able to use |
77 | ffedf64d | Michele Tartara | multiple ones, and has to deal with doing different things, thus making |
78 | ffedf64d | Michele Tartara | sometimes unclear which daemon is responsible for performing a specific task. |
79 | ffedf64d | Michele Tartara | |
80 | ffedf64d | Michele Tartara | Also, with the current configuration, jobs are managed by the master daemon |
81 | ffedf64d | Michele Tartara | using python threads. This makes terminating a job after it has started a |
82 | ffedf64d | Michele Tartara | difficult operation, and it is the main reason why this is not possible yet. |
83 | ffedf64d | Michele Tartara | |
84 | ffedf64d | Michele Tartara | The master daemon currently has too many different tasks, that could be handled |
85 | ffedf64d | Michele Tartara | better if split among different daemons. |
86 | ffedf64d | Michele Tartara | |
87 | ffedf64d | Michele Tartara | |
88 | ffedf64d | Michele Tartara | Proposed changes |
89 | ffedf64d | Michele Tartara | ================ |
90 | ffedf64d | Michele Tartara | |
91 | ffedf64d | Michele Tartara | In order to improve on the current situation, a new daemon subdivision is |
92 | ffedf64d | Michele Tartara | proposed, and presented hereafter. |
93 | ffedf64d | Michele Tartara | |
94 | ffedf64d | Michele Tartara | .. digraph:: "new-daemons-structure" |
95 | ffedf64d | Michele Tartara | |
96 | ffedf64d | Michele Tartara | {rank=same; RConfD LuxiD;} |
97 | ffedf64d | Michele Tartara | {rank=same; Jobs rconfigdata;} |
98 | ffedf64d | Michele Tartara | node [shape=box] |
99 | ffedf64d | Michele Tartara | RapiD [label="RapiD [M]"] |
100 | ffedf64d | Michele Tartara | LuxiD [label="LuxiD [M]"] |
101 | ffedf64d | Michele Tartara | WConfD [label="WConfD [M]"] |
102 | ffedf64d | Michele Tartara | Jobs [label="Jobs [M]"] |
103 | ffedf64d | Michele Tartara | RConfD [label="RConfD [MC]"] |
104 | ffedf64d | Michele Tartara | MonD [label="MonD [All]"] |
105 | ffedf64d | Michele Tartara | NodeD [label="NodeD [All]"] |
106 | ffedf64d | Michele Tartara | Clients [label="gnt-*\nclients [M]"] |
107 | ffedf64d | Michele Tartara | p1 [shape=none, label=""] |
108 | ffedf64d | Michele Tartara | p2 [shape=none, label=""] |
109 | ffedf64d | Michele Tartara | p3 [shape=none, label=""] |
110 | ffedf64d | Michele Tartara | p4 [shape=none, label=""] |
111 | ffedf64d | Michele Tartara | configdata [shape=none, label="config.data"] |
112 | ffedf64d | Michele Tartara | rconfigdata [shape=none, label="config.data\n[MC copy]"] |
113 | ffedf64d | Michele Tartara | locksdata [shape=none, label="locks.data"] |
114 | ffedf64d | Michele Tartara | |
115 | ffedf64d | Michele Tartara | RapiD -> LuxiD [label="LUXI"] |
116 | ffedf64d | Michele Tartara | LuxiD -> WConfD [label="WConfD\nproto"] |
117 | ffedf64d | Michele Tartara | LuxiD -> Jobs [label="fork/exec"] |
118 | ffedf64d | Michele Tartara | Jobs -> WConfD [label="WConfD\nproto"] |
119 | ffedf64d | Michele Tartara | Jobs -> NodeD [label="RPC"] |
120 | ffedf64d | Michele Tartara | LuxiD -> NodeD [label="RPC"] |
121 | ffedf64d | Michele Tartara | rconfigdata -> RConfD |
122 | ffedf64d | Michele Tartara | configdata -> rconfigdata [label="sync via\nNodeD RPC"] |
123 | ffedf64d | Michele Tartara | WConfD -> NodeD [label="RPC"] |
124 | ffedf64d | Michele Tartara | WConfD -> configdata |
125 | ffedf64d | Michele Tartara | WConfD -> locksdata |
126 | ffedf64d | Michele Tartara | MonD -> RConfD [label="RConfD\nproto"] |
127 | ffedf64d | Michele Tartara | Clients -> LuxiD [label="LUXI"] |
128 | ffedf64d | Michele Tartara | p1 -> MonD [label="MonD proto"] |
129 | ffedf64d | Michele Tartara | p2 -> RapiD [label="RAPI"] |
130 | ffedf64d | Michele Tartara | p3 -> RConfD [label="RConfD\nproto"] |
131 | ffedf64d | Michele Tartara | p4 -> Clients [label="CLI"] |
132 | ffedf64d | Michele Tartara | |
133 | ffedf64d | Michele Tartara | ``LUXI daemon (LuxiD)`` |
134 | ffedf64d | Michele Tartara | It will be written in Haskell. It will run on the master node and it will be |
135 | ffedf64d | Michele Tartara | the only LUXI server, replying to all the LUXI queries. These includes both |
136 | ffedf64d | Michele Tartara | the queries about the live configuration of the cluster, previously served by |
137 | ffedf64d | Michele Tartara | QueryD, and the commands actually changing the status of the cluster by |
138 | ffedf64d | Michele Tartara | submitting jobs. Therefore, this daemon will also be the one responsible with |
139 | ffedf64d | Michele Tartara | managing the job queue. When a job needs to be executed, the LuxiD will spawn |
140 | ffedf64d | Michele Tartara | a separate process tasked with the execution of that specific job, thus making |
141 | ffedf64d | Michele Tartara | it easier to terminate the job itself, if needeed. When a job requires locks, |
142 | ffedf64d | Michele Tartara | LuxiD will request them from WConfD. |
143 | ffedf64d | Michele Tartara | In order to keep availability of the cluster in case of failure of the master |
144 | ffedf64d | Michele Tartara | node, LuxiD will replicate the job queue to the other master candidates, by |
145 | ffedf64d | Michele Tartara | RPCs to the NodeD running there (the choice of RPCs for this task might be |
146 | ffedf64d | Michele Tartara | reviewed at a second time, after implementing this design). |
147 | ffedf64d | Michele Tartara | |
148 | ffedf64d | Michele Tartara | ``Configuration management daemon (WConfD)`` |
149 | ffedf64d | Michele Tartara | It will run on the master node and it will be responsible for the management |
150 | ffedf64d | Michele Tartara | of the authoritative copy of the cluster configuration (that is, it will be |
151 | ffedf64d | Michele Tartara | the daemon actually modifying the ``config.data`` file). All the requests of |
152 | ffedf64d | Michele Tartara | configuration changes will have to pass through this daemon, and will be |
153 | ffedf64d | Michele Tartara | performed using a LUXI-like protocol ("WConfD proto" in the graph. The exact |
154 | ffedf64d | Michele Tartara | protocol will be defined in the separate design document that will detail the |
155 | ffedf64d | Michele Tartara | WConfD separation). Having a single point of configuration management will |
156 | ffedf64d | Michele Tartara | also allow Ganeti to get rid of possible race conditions due to concurrent |
157 | ffedf64d | Michele Tartara | modifications of the configuration. When the configuration is updated, it |
158 | ffedf64d | Michele Tartara | will have to push the received changes to the other master candidates, via |
159 | ffedf64d | Michele Tartara | RPCs, so that RConfD daemons and (in case of a failure on the master node) |
160 | ffedf64d | Michele Tartara | the WConfD daemon on the new master can access an up-to-date version of it |
161 | ffedf64d | Michele Tartara | (the choice of RPCs for this task might be reviewed at a second time). This |
162 | ffedf64d | Michele Tartara | daemon will also be the one responsible for managing the locks, granting them |
163 | ffedf64d | Michele Tartara | to the jobs requesting them, and taking care of freeing them up if the jobs |
164 | ffedf64d | Michele Tartara | holding them crash or are terminated before releasing them. In order to do |
165 | ffedf64d | Michele Tartara | this, each job, after being spawned by LuxiD, will open a local unix socket |
166 | ffedf64d | Michele Tartara | that will be used to communicate with it, and will be destroyed when the job |
167 | ffedf64d | Michele Tartara | terminates. LuxiD will be able to check, after a timeout, whether the job is |
168 | ffedf64d | Michele Tartara | still running by connecting here, and to ask WConfD to forcefully remove the |
169 | ffedf64d | Michele Tartara | locks if the socket is closed. |
170 | ffedf64d | Michele Tartara | Also, WConfD should hold a serialized list of the locks and their owners in a |
171 | ffedf64d | Michele Tartara | file (``locks.data``), so that it can keep track of their status in case it |
172 | ffedf64d | Michele Tartara | crashes and needs to be restarted (by asking LuxiD which of them are still |
173 | ffedf64d | Michele Tartara | running). |
174 | ffedf64d | Michele Tartara | Interaction with this daemon will be performed using Unix sockets. |
175 | ffedf64d | Michele Tartara | |
176 | ffedf64d | Michele Tartara | ``Configuration query daemon (RConfD)`` |
177 | ffedf64d | Michele Tartara | It is written in Haskell, and it corresponds to the old ConfD. It will run on |
178 | ffedf64d | Michele Tartara | all the master candidates and it will serve information about the the static |
179 | ffedf64d | Michele Tartara | configuration of the cluster (the one contained in ``config.data``). The |
180 | ffedf64d | Michele Tartara | provided information will be highly available (as in: a response will be |
181 | ffedf64d | Michele Tartara | available as long as a stable-enough connection between the client and at |
182 | ffedf64d | Michele Tartara | least one working master candidate is available) and its freshness will be |
183 | ffedf64d | Michele Tartara | best effort (the most recent reply from any of the master candidates will be |
184 | ffedf64d | Michele Tartara | returned, but it might still be older than the one available through WConfD). |
185 | ffedf64d | Michele Tartara | The information will be served through the ConfD protocol. |
186 | ffedf64d | Michele Tartara | |
187 | ffedf64d | Michele Tartara | ``Rapi daemon (RapiD)`` |
188 | ffedf64d | Michele Tartara | It remains basically unchanged, with the only difference that all of its LUXI |
189 | ffedf64d | Michele Tartara | query are directed towards LuxiD instead of being split between MasterD and |
190 | ffedf64d | Michele Tartara | QueryD. |
191 | ffedf64d | Michele Tartara | |
192 | ffedf64d | Michele Tartara | ``Monitoring daemon (MonD)`` |
193 | ffedf64d | Michele Tartara | It remains unaffected by the changes in this design document. It will just get |
194 | ffedf64d | Michele Tartara | some of the data it needs from RConfD instead of the old ConfD, but the |
195 | ffedf64d | Michele Tartara | interfaces of the two are identical. |
196 | ffedf64d | Michele Tartara | |
197 | ffedf64d | Michele Tartara | ``Node daemon (NodeD)`` |
198 | ffedf64d | Michele Tartara | It remains unaffected by the changes proposed in the design document. The only |
199 | ffedf64d | Michele Tartara | difference being that it will receive its RPCs from LuxiD (for job queue |
200 | ffedf64d | Michele Tartara | replication), from WConfD (for configuration replication) and for the |
201 | ffedf64d | Michele Tartara | processes executing single jobs (for all the operations to be performed by |
202 | ffedf64d | Michele Tartara | nodes) instead of receiving them just from MasterD. |
203 | ffedf64d | Michele Tartara | |
204 | ffedf64d | Michele Tartara | This restructuring will allow us to reorganize and improve the codebase, |
205 | ffedf64d | Michele Tartara | introducing cleaner interfaces and giving well defined and more restricted tasks |
206 | ffedf64d | Michele Tartara | to each daemon. |
207 | ffedf64d | Michele Tartara | |
208 | ffedf64d | Michele Tartara | Furthermore, having more well-defined interfaces will allow us to have easier |
209 | ffedf64d | Michele Tartara | upgrade procedures, and to work towards the possibility of upgrading single |
210 | ffedf64d | Michele Tartara | components of a cluster one at a time, without the need for immediately |
211 | ffedf64d | Michele Tartara | upgrading the entire cluster in a single step. |
212 | ffedf64d | Michele Tartara | |
213 | ffedf64d | Michele Tartara | |
214 | ffedf64d | Michele Tartara | Implementation |
215 | ffedf64d | Michele Tartara | ============== |
216 | ffedf64d | Michele Tartara | |
217 | ffedf64d | Michele Tartara | While performing this refactoring, we aim to increase the amount of |
218 | ffedf64d | Michele Tartara | Haskell code, thus benefiting from the additional type safety provided by its |
219 | ffedf64d | Michele Tartara | wide compile-time checks. In particular, all the job queue management and the |
220 | ffedf64d | Michele Tartara | configuration management daemon will be written in Haskell, taking over the role |
221 | ffedf64d | Michele Tartara | currently fulfilled by Python code executed as part of MasterD. |
222 | ffedf64d | Michele Tartara | |
223 | ffedf64d | Michele Tartara | The changes describe by this design document are quite extensive, therefore they |
224 | ffedf64d | Michele Tartara | will not be implemented all at the same time, but through a sequence of steps, |
225 | ffedf64d | Michele Tartara | leaving the codebase in a consistent and usable state. |
226 | ffedf64d | Michele Tartara | |
227 | ffedf64d | Michele Tartara | #. Rename QueryD to LuxiD. |
228 | ffedf64d | Michele Tartara | A part of LuxiD, the one replying to configuration |
229 | ffedf64d | Michele Tartara | queries including live information about the system, already exists in the |
230 | ffedf64d | Michele Tartara | form of QueryD. This is being renamed to LuxiD, and will form the first part |
231 | ffedf64d | Michele Tartara | of the new daemon. NB: this is happening starting from Ganeti 2.8. At the |
232 | ffedf64d | Michele Tartara | beginning, only the already existing queries will be replied to by LuxiD. |
233 | ffedf64d | Michele Tartara | More queries will be implemented in the next versions. |
234 | ffedf64d | Michele Tartara | |
235 | ffedf64d | Michele Tartara | #. Let LuxiD be the interface for the queries and MasterD be their executor. |
236 | ffedf64d | Michele Tartara | Currently, MasterD is the only responsible for receiving and executing LUXI |
237 | ffedf64d | Michele Tartara | queries, and for managing the jobs they create. |
238 | ffedf64d | Michele Tartara | Receiving the queries and managing the job queue will be extracted from |
239 | ffedf64d | Michele Tartara | MasterD into LuxiD. |
240 | ffedf64d | Michele Tartara | Actually executing jobs will still be done by MasterD, that contains all the |
241 | ffedf64d | Michele Tartara | logic for doing that and for properly managing locks and the configuration. |
242 | ffedf64d | Michele Tartara | A separate design document will detail how the system will decide which jobs |
243 | ffedf64d | Michele Tartara | to send over for execution, and how to rate-limit them. |
244 | ffedf64d | Michele Tartara | |
245 | ffedf64d | Michele Tartara | #. Extract WConfD from MasterD. |
246 | ffedf64d | Michele Tartara | The logic for managing the configuration file is factored out to the |
247 | ffedf64d | Michele Tartara | dedicated WConfD daemon. All configuration changes, currently executed |
248 | ffedf64d | Michele Tartara | directly by MasterD, will be changed to be IPC requests sent to the new |
249 | ffedf64d | Michele Tartara | daemon. |
250 | ffedf64d | Michele Tartara | |
251 | ffedf64d | Michele Tartara | #. Extract locking management from MasterD. |
252 | ffedf64d | Michele Tartara | The logic for managing and granting locks is extracted to WConfD as well. |
253 | ffedf64d | Michele Tartara | Locks will not be taken directly anymore, but asked via IPC to WConfD. |
254 | ffedf64d | Michele Tartara | This step can be executed on its own or at the same time as the previous one. |
255 | ffedf64d | Michele Tartara | |
256 | ffedf64d | Michele Tartara | #. Jobs are executed as processes. |
257 | ffedf64d | Michele Tartara | The logic for running jobs is rewritten so that each job can be managed by an |
258 | ffedf64d | Michele Tartara | independent process. LuxiD will spawn a new (Python) process for every single |
259 | ffedf64d | Michele Tartara | job. The RPCs will remain unchanged, and the LU code will stay as is as much |
260 | ffedf64d | Michele Tartara | as possible. |
261 | ffedf64d | Michele Tartara | MasterD will cease to exist as a deamon on its own at this point, but not |
262 | ffedf64d | Michele Tartara | before. |
263 | ffedf64d | Michele Tartara | |
264 | ffedf64d | Michele Tartara | Further considerations |
265 | ffedf64d | Michele Tartara | ====================== |
266 | ffedf64d | Michele Tartara | |
267 | ffedf64d | Michele Tartara | There is a possibility that a job will finish performing its task while LuxiD |
268 | ffedf64d | Michele Tartara | and/or WConfD will not be available. |
269 | ffedf64d | Michele Tartara | In order to deal with this situation, each job will write the results of its |
270 | ffedf64d | Michele Tartara | execution on a file. The name of this file will be known to LuxiD before |
271 | ffedf64d | Michele Tartara | starting the job, and will be stored together with the job ID, and the |
272 | ffedf64d | Michele Tartara | name of the job-unique socket. |
273 | ffedf64d | Michele Tartara | |
274 | ffedf64d | Michele Tartara | The job, upon ending its execution, will signal LuxiD (through the socket), so |
275 | ffedf64d | Michele Tartara | that it can read the result of the execution and release the locks as needed. |
276 | ffedf64d | Michele Tartara | |
277 | ffedf64d | Michele Tartara | In case LuxiD is not available at that time, the job will just terminate without |
278 | ffedf64d | Michele Tartara | signalling it, and writing the results on file as usual. When a new LuxiD |
279 | ffedf64d | Michele Tartara | becomes available, it will have the most up-to-date list of running jobs |
280 | ffedf64d | Michele Tartara | (received via replication from the former LuxiD), and go through it, cleaning up |
281 | ffedf64d | Michele Tartara | all the terminated jobs. |
282 | ffedf64d | Michele Tartara | |
283 | ffedf64d | Michele Tartara | |
284 | ffedf64d | Michele Tartara | .. vim: set textwidth=72 : |
285 | ffedf64d | Michele Tartara | .. Local Variables: |
286 | ffedf64d | Michele Tartara | .. mode: rst |
287 | ffedf64d | Michele Tartara | .. fill-column: 72 |
288 | ffedf64d | Michele Tartara | .. End: |