If the real use of this work item is to use: - Only one bookie - Bookie is the metadata server too
Maybe it is ok. But I am worried about the *magnitude* of the code refactor to accommodate this. I really like the idea of finding an alternate to ZK, but I think we need to have that direction ironed out before rushing with the first step without knowing what the next step is. Again, I am worried about the code refactor and resulting regression. Removing ZK is one part, but changing from centralized metadata server (ZK/etcd kind) model to distributing metadata across bookies in the cluster is a HUGE leap. We are looking at implementing our own version of Paxos. It is possible I am not understanding the full intent here and would love to have more discussion. Can we have more discussion on Thursday call again? Thanks, JV On Sat, Sep 16, 2017 at 3:17 AM, Enrico Olivelli <eolive...@gmail.com> wrote: > Yep > Thank you Jia > > Enrico > > On sab 16 set 2017, 12:10 Jia Zhai <zhaiji...@gmail.com> wrote: > > > Since there is not objective. would like to make this BP approved. > > > > On Wed, Sep 13, 2017 at 4:24 PM, Sijie Guo <guosi...@gmail.com> wrote: > > > > > On Wed, Sep 13, 2017 at 1:18 AM, Enrico Olivelli <eolive...@gmail.com> > > > wrote: > > > > > > > 2017-09-13 10:10 GMT+02:00 Sijie Guo <guosi...@gmail.com>: > > > > > > > > > On Wed, Sep 13, 2017 at 12:16 AM, Enrico Olivelli < > > eolive...@gmail.com > > > > > > > > > wrote: > > > > > > > > > > > I think that this is a good direction to go. > > > > > > > > > > > > I believe to the reasons about ZK in huge systems even it is not > my > > > > case > > > > > so > > > > > > I cannot add comments on this usecase. > > > > > > > > > > > > I am fine with direction as long as we are still going to support > > > > > > ZooKeeper. > > > > > > BookKeeper is in the Hadoop / ZooKeeper ecosystem and several > > > products > > > > > rely > > > > > > on ZK too, for instance in my systems it is usual to have > > > > > > BookKeeper/Kafka/HBase/Majordodo.... and so I am not going to > live > > > > > > without > > > > > > zookeeper in the short/mid term. > > > > > > > > > > > > I am really OK in dropping ZK because for "simple" systems in > fact > > > when > > > > > you > > > > > > need only BK having the burden of setting up a zookeeper server > is > > > > weird > > > > > > for customers. I usually re-distribute BK + ZK with my > applications > > > and > > > > > we > > > > > > are talking about little clusters of up to 10 machines. > > > > > > > > > > > > > > > > Just to clarify - we are not dropping ZK here. we are just > proposing > > to > > > > > have a ledger manager implementation that doesn't depend on > zookeeper > > > > > directly. > > > > > We are not modifying any existing ledger manager implementation. > > > > > > > > > > > > > > > > > Yep, we are on the same page > > > > for this proposal the bookie will be a sort of "proxy" between the > > client > > > > and the actual ledger manager implementation which will "live" inside > > the > > > > bookie > > > > it is only a new ledger manager to be used in clients, this ledger > > > manager > > > > will issue RPCs (or kind of "streaming" RPCs) to a list of bookies > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > The direction on this proposal is OK for me and it is very like > the > > > > work > > > > > I > > > > > > was starting about "standalone mode". > > > > > > > > > > > > > > > > I think it will be very easy to support the case of having a > single > > > > > bookie > > > > > > with this approach or even client+ bookie in the same JVM, > > > > > > Having multiple bookies will make us to add some other > coordination > > > > > > facility between bookies, I would like to know if there is > already > > > some > > > > > > idea about this, are we going to use another product like > > > etcd,jgroups > > > > or > > > > > > implement our own coordination protocol ? > > > > > > > > > > > > > > > we are not replacing A with B, even with zookeeper. the ledger > > > management > > > > > is already abstracted in interfaces. > > > > > the users can use whatever system they prefer as the metadata > store. > > > > > > > > > > our direction is to provide an option to store metadata as well as > > data > > > > in > > > > > bookies. so in this option, there is no external metadata storage > > > needed. > > > > > > > > > > > > > Sorry. Maybe my curiosity is not clear. > > > > If you have multiple bookies and each bookie holds its own version of > > > > metadata, how do you coordinate them ? which will be the source of > > truth > > > ? > > > > Maybe we should start a new email thread in the future to talk about > > > > "alternative distributed metadata storages" > > > > > > > > > > It is out of the scope of this BP. We will have a next BP to cover this > > > part. > > > > > > > > > > > > > > > > > > > > Any way the meaning and the scope of the proposal is clear to me and > I > > am > > > > really OK with it, I hope it will get soon approved > > > > > > > > -- Enrico > > > > > > > > > > > > > > > > > > > > > > > > ZK is simple but it very > > > > > > effective. > > > > > > > > > > Maybe we could help the ZK community to move forward and resolve > > > > > > the problems we are bringing to light > > > > > > > > > > > > > > > > > > Enrico > > > > > > > > > > > > > > > > > > 2017-09-13 3:15 GMT+02:00 Jia Zhai <zhaiji...@gmail.com>: > > > > > > > > > > > > > Any thoughts or comments > > > > > > > :) > > > > > > > > > > > > > > Thanks a lot. > > > > > > > -Jia > > > > > > > > > > > > > > On Tue, Sep 12, 2017 at 4:30 PM, Jia Zhai <zhaiji...@gmail.com > > > > > > wrote: > > > > > > > > > > > > > > > This blog: https://bitworks.software/ > > > > blog/en/2017-07-12-replicated- > > > > > > > > scalable-commitlog-with-apachebookkeeper.html, which also > refer > > > a > > > > > > little > > > > > > > > the limitation of zookeeper in bookkeeper > > > > > > > > > > > > > > > > On Thu, Sep 7, 2017 at 9:45 AM, Jia Zhai < > zhaiji...@gmail.com> > > > > > wrote: > > > > > > > > > > > > > > > >> 👍. Thanks a lot for the suggestions and feed back. > > > > > > > >> > > > > > > > >> On Thu, Sep 7, 2017 at 4:24 AM, Sijie Guo < > guosi...@gmail.com > > > > > > > > wrote: > > > > > > > >> > > > > > > > >>> On Wed, Sep 6, 2017 at 1:07 PM, Enrico Olivelli < > > > > > eolive...@gmail.com > > > > > > > > > > > > > > >>> wrote: > > > > > > > >>> > > > > > > > >>> > Off topic curiosity... Jia and Sijie, do you think we are > > > going > > > > > to > > > > > > > >>> drop ZK > > > > > > > >>> > from DL too? > > > > > > > >>> > > > > > > > > >>> > > > > > > > >>> Yes. That's the goal - 1) for large deployment, we are > trying > > > to > > > > > > > overcome > > > > > > > >>> the limitation of zookeeper; 2) for smaller deployments, it > > > will > > > > > make > > > > > > > >>> deployment much easier, you just need to deploy a cluster > of > > > > > bookies. > > > > > > > >>> once > > > > > > > >>> it is done, you can use ledger api or log stream api to > > access > > > > the > > > > > > > >>> bookkeeper cluster. > > > > > > > >>> > > > > > > > >>> Both DL and BK are metadata storage pluggable. They have > very > > > > clear > > > > > > > >>> interfaces on defining metadata operations. So it is > > > > > straightforward > > > > > > to > > > > > > > >>> use > > > > > > > >>> a different metadata storage. > > > > > > > >>> > > > > > > > >>> > > > > > > > >>> > Enrico > > > > > > > >>> > > > > > > > > >>> > On mer 6 set 2017, 19:51 Enrico Olivelli < > > > eolive...@gmail.com> > > > > > > > wrote: > > > > > > > >>> > > > > > > > > >>> > > > > > > > > > >>> > > > > > > > > > >>> > > On mer 6 set 2017, 18:25 Sijie Guo <guosi...@gmail.com > > > > > > wrote: > > > > > > > >>> > > > > > > > > > >>> > >> On Sep 6, 2017 4:57 AM, "Enrico Olivelli" < > > > > > eolive...@gmail.com> > > > > > > > >>> wrote: > > > > > > > >>> > >> > > > > > > > >>> > >> Thank you Sijie and Jia for your comments and > > > explanations, > > > > > > > >>> > >> answers inline > > > > > > > >>> > >> > > > > > > > >>> > >> 2017-09-06 2:23 GMT+02:00 Jia Zhai < > zhaiji...@gmail.com > > >: > > > > > > > >>> > >> > > > > > > > >>> > >> > Thanks a lot Enrico and Sijie for your comments and > > > > > > information > > > > > > > on > > > > > > > >>> > this. > > > > > > > >>> > >> > > > > > > > > >>> > >> > On Tue, Sep 5, 2017 at 9:31 PM, Enrico Olivelli < > > > > > > > >>> eolive...@gmail.com> > > > > > > > >>> > >> > wrote: > > > > > > > >>> > >> > > > > > > > > >>> > >> > > Great to see you working on this ! > > > > > > > >>> > >> > > I would be great to have such feature, as it is > the > > > > first > > > > > > step > > > > > > > >>> to a > > > > > > > >>> > >> > > 'standalone' BookKeeper mode > > > > > > > >>> > >> > > > > > > > > > >>> > >> > > Some complementary ideas/first look questions: > > > > > > > >>> > >> > > - the document does not talk about security, IMHO > we > > > > have > > > > > at > > > > > > > >>> least > > > > > > > >>> > to > > > > > > > >>> > >> > cover > > > > > > > >>> > >> > > authentication and TLS, it would be great to > > leverage > > > > > > existing > > > > > > > >>> > >> > AuthPlugins, > > > > > > > >>> > >> > > as they are based on exchanging byte[] (as SASL > > wants) > > > > > > > >>> > >> > > > > > > > > > >>> > >> > [Jia] It is a good idea. We left the security part > for > > > now > > > > > > for a > > > > > > > >>> few > > > > > > > >>> > >> > reasons. 1) Make this BP more focus on removing > > > zookeeper > > > > > > > >>> dependencies > > > > > > > >>> > >> from > > > > > > > >>> > >> > client. 2) It is introduced as a separated > > > implementation > > > > of > > > > > > > >>> existing > > > > > > > >>> > >> > interfaces. So it won’t impact existing security > > story. > > > > > And > > > > > > > for > > > > > > > >>> > sure, > > > > > > > >>> > >> We > > > > > > > >>> > >> > will add the security part later after this. > > > > > > > >>> > >> > > > > > > > > >>> > >> > > > > > > > >>> > >> > > > > > > > >>> > >> I am fine, I am only afraid that we won't be able to > > > support > > > > > it > > > > > > in > > > > > > > >>> the > > > > > > > >>> > >> (near) future, > > > > > > > >>> > >> maybe you could just only cite the security story and > > add > > > > some > > > > > > > >>> reference > > > > > > > >>> > >> to > > > > > > > >>> > >> how we would deal with it in future > > > > > > > >>> > >> > > > > > > > >>> > >> > > > > > > > >>> > >> The new ledger manager will be first marked as > > > experimental, > > > > > > until > > > > > > > >>> it is > > > > > > > >>> > >> stable and have security feature. > > > > > > > >>> > >> > > > > > > > >>> > >> How does that sound? > > > > > > > >>> > >> > > > > > > > >>> > > > > > > > > > >>> > > Ok > > > > > > > >>> > > > > > > > > > >>> > >> > > > > > > > >>> > >> > > > > > > > >>> > >> > > > > > > > >>> > >> > > > > > > > > >>> > >> > - do we have some kind of "bootstrap servers list" > > > > > > configuration > > > > > > > >>> > option > > > > > > > >>> > >> ? > > > > > > > >>> > >> > > the list should be complete or just a subset of > > > bookies > > > > ? > > > > > at > > > > > > > >>> > >> connection > > > > > > > >>> > >> > the > > > > > > > >>> > >> > > client could discover the list of other bookies > > > > > > > >>> > >> > > > > > > > > > >>> > >> > [Jia] Yes, we will have a `clientBootstrapBookies` > > > > settings > > > > > in > > > > > > > the > > > > > > > >>> > >> server > > > > > > > >>> > >> > set. It can be a list of bookies or just simple a > DNS > > > over > > > > > the > > > > > > > >>> > bookies. > > > > > > > >>> > >> > Will add this to the BP > > > > > > > >>> > >> > > > > > > > > >>> > >> > - will the client connect to only one bookie at a > > time ? > > > > how > > > > > > we > > > > > > > >>> will > > > > > > > >>> > >> deal > > > > > > > >>> > >> > > with errors ? > > > > > > > >>> > >> > > > > > > > > > >>> > >> > [Jia] It will connect the the list of bootstrap > > servers. > > > > > gPRC > > > > > > > will > > > > > > > >>> > load > > > > > > > >>> > >> > balance the requests and manage the connection > errors. > > > > > > > >>> > >> > > > > > > > > >>> > >> > - should the bookie write on ZK metadata its gRPC > > > endpoint > > > > > > info > > > > > > > ? > > > > > > > >>> > (this > > > > > > > >>> > >> > > will be useful for a bookie to tell about other > > > bookies > > > > to > > > > > > the > > > > > > > >>> > >> connected > > > > > > > >>> > >> > > clients) > > > > > > > >>> > >> > > > > > > > > > >>> > >> > [Jia]No, it won’t. We don’t see a strong reason to > add > > > it. > > > > > > > >>> Especially > > > > > > > >>> > >> > eventually we may eliminate zookeeper completely. > > > > > > > >>> > >> > It can be a fixed port `3281`, or in a > scheduler-based > > > > > > > >>> environment, it > > > > > > > >>> > >> is > > > > > > > >>> > >> > very easy to have a load balancer sitting in front > of > > > > those > > > > > > > >>> bookies. > > > > > > > >>> > >> > > > > > > > > >>> > >> > > > > > > > >>> > >> I think a fixed port is not a good way. > > > > > > > >>> > >> You will not be able to run more than one bookie on a > > > single > > > > > > host. > > > > > > > >>> > >> > > > > > > > >>> > >> We should support: > > > > > > > >>> > >> - configurable port > > > > > > > >>> > >> - ephemeral port for tests > > > > > > > >>> > >> > > > > > > > >>> > >> > > > > > > > >>> > >> I think what Jia means is a configurable port, but it > > is a > > > > > > > >>> relatively > > > > > > > >>> > >> fixed > > > > > > > >>> > >> port, which client doesn't discover this port from > > > > zookeeper. > > > > > > > >>> > >> > > > > > > > >>> > > > > > > > > > >>> > > Very good > > > > > > > >>> > > > > > > > > > >>> > >> > > > > > > > >>> > >> > > > > > > > >>> > >> Ideally I would like to have the local transport > option, > > > in > > > > > > order > > > > > > > to > > > > > > > >>> > have > > > > > > > >>> > >> a > > > > > > > >>> > >> single JVM, but this is not a blocker problem, as we > are > > > > > running > > > > > > > >>> gRPC on > > > > > > > >>> > >> netty it should be feasible or we can create some kind > > of > > > > > > > >>> short-circut > > > > > > > >>> > >> between the client and the Bookie > > > > > > > >>> > >> > > > > > > > >>> > >> > > > > > > > >>> > >> GRPC supports inprocess channel. So you don't need to > > use > > > > the > > > > > > low > > > > > > > >>> level > > > > > > > >>> > >> netty settings. > > > > > > > >>> > >> > > > > > > > >>> > > > > > > > > > >>> > > Great > > > > > > > >>> > > > > > > > > > >>> > > So it sounds all good to me thanks > > > > > > > >>> > > > > > > > > > >>> > > Enrico > > > > > > > >>> > > > > > > > > > >>> > > > > > > > > > >>> > >> > > > > > > > >>> > >> I am OK for not writing this to the bookie metadata, > > > leaving > > > > > up > > > > > > to > > > > > > > >>> the > > > > > > > >>> > >> client have a configured list of bookies enabled to > > > metadata > > > > > > > >>> operations > > > > > > > >>> > >> > > > > > > > >>> > >> > > > > > > > >>> > >> > > > > > > > >>> > >> > > > > > > > >>> > >> > > > > > > > > >>> > >> > - the bookie will be somehow a proxy for zookeeper, > I > > > > think > > > > > > that > > > > > > > >>> the > > > > > > > >>> > >> > > 'watch' part is the more complex, we will have to > > deal > > > > > with > > > > > > > >>> > >> > reconnections, > > > > > > > >>> > >> > > errors....maybe it is worth to write more detail > > about > > > > > this > > > > > > > >>> > >> > > > > > > > > > >>> > >> > [Jia] The `watch` API is using the `streaming` rpc > in > > > > gRPC. > > > > > It > > > > > > > is > > > > > > > >>> a > > > > > > > >>> > >> > straightforward proxy behavior, if a connection is > > > broken, > > > > > the > > > > > > > >>> client > > > > > > > >>> > >> will > > > > > > > >>> > >> > simply retry on watching again. > > > > > > > >>> > >> > > > > > > > > >>> > >> > > > > > > > > >>> > >> > > Minor issues: > > > > > > > >>> > >> > > - Maybe you can consider using ledgerId and not > > > > ledger_id, > > > > > > > like > > > > > > > >>> in > > > > > > > >>> > >> > > LedgerMetadataFormat we are using lastEntryId > > > > > > > >>> > >> > > > > > > > > > >>> > >> > [Jia] Thanks, It is a protobuf style. The protobuf > > will > > > > > > convert > > > > > > > >>> > >> `ledger_id` > > > > > > > >>> > >> > to `ledgerId`. We don’t need to worry about this. > > > > > > > >>> > >> > > > > > > > > >>> > >> > > > > > > > >>> > >> got it, thanks > > > > > > > >>> > >> > > > > > > > >>> > >> > > > > > > > >>> > >> > > > > > > > > >>> > >> > > > > > > > > >>> > >> > > -In the "motivation" part you write that the fact > > the > > > > > having > > > > > > > >>> more > > > > > > > >>> > >> clients > > > > > > > >>> > >> > > than the number of bookies would be a problem for > > > > > zookeeper, > > > > > > > >>> > actually > > > > > > > >>> > >> > > zookeeper is very good at dealing with a huge > number > > > of > > > > > > > clients. > > > > > > > >>> > >> > Actually I > > > > > > > >>> > >> > > am always running clusters with 3-5 bookies and > > 10-100 > > > > > > writing > > > > > > > >>> > clients > > > > > > > >>> > >> > and > > > > > > > >>> > >> > > this has never given troubles > > > > > > > >>> > >> > > > > > > > > >>> > >> > [Jia] :) Seems “10-100 writing clients” is not “a > huge > > > > > number > > > > > > of > > > > > > > >>> > >> clients”. > > > > > > > >>> > >> > > > > > > > > >>> > >> > > > > > > > >>> > >> OK, I agree with you an Sijie, I have no experience of > > > > larger > > > > > > > >>> clusters > > > > > > > >>> > >> > > > > > > > >>> > >> > > > > > > > >>> > >> > > > > > > > > >>> > >> > > > > > > > > > >>> > >> > > > > > > > > >>> > >> > > > > > > > > >>> > >> > > > > > > > > >>> > >> > > Future: > > > > > > > >>> > >> > > - as bookies will be proxies maybe we should take > > care > > > > not > > > > > > to > > > > > > > >>> > >> overwhelm > > > > > > > >>> > >> a > > > > > > > >>> > >> > > bookie with too many clients > > > > > > > >>> > >> > > > > > > > > > >>> > >> > [Jia] First, gRPC is based on Netty, the protocol is > > > > http2, > > > > > so > > > > > > > the > > > > > > > >>> > >> > connection is multiplexed. We don’t need to worry > > about > > > > > > > connection > > > > > > > >>> > >> count. > > > > > > > >>> > >> > Second, all the bookies are treated equally for the > > > > metadata > > > > > > > >>> > operations, > > > > > > > >>> > >> > gRPC will load balancing the requests across the > > > bookies. > > > > We > > > > > > > don’t > > > > > > > >>> > need > > > > > > > >>> > >> to > > > > > > > >>> > >> > worry about some bookies are overwhelmed. > > > > > > > >>> > >> > > > > > > > > >>> > >> > > > > > > > >>> > >> gRPC sounds great > > > > > > > >>> > >> > > > > > > > >>> > >> > > > > > > > >>> > >> > > > > > > > > >>> > >> > > > > > > > > >>> > >> > > - iteration on ledgers, sometimes the clients > > > enumerates > > > > > > > >>> ledgers but > > > > > > > >>> > >> it > > > > > > > >>> > >> > is > > > > > > > >>> > >> > > not interested in having all of them, as we are > > using > > > > the > > > > > > > >>> bookie as > > > > > > > >>> > >> proxy > > > > > > > >>> > >> > > maybe some kind of "filter" (at least on custom > > > > metadata) > > > > > > > would > > > > > > > >>> be > > > > > > > >>> > >> create > > > > > > > >>> > >> > > to limit the number of returned items. Other > point I > > > > don't > > > > > > > know > > > > > > > >>> gRPC > > > > > > > >>> > >> but > > > > > > > >>> > >> > it > > > > > > > >>> > >> > > does not seems to be very clear how to 'stop' the > > > > > iteration > > > > > > > >>> > >> > > > > > > > > > >>> > >> > [Jia] Thanks, We can add it later. For now, we would > > > like > > > > to > > > > > > > >>> focus on > > > > > > > >>> > >> > adding the features the ledger manager needs. > > > > > > > >>> > >> > > > > > > > > >>> > >> > > > > > > > >>> > >> Yup > > > > > > > >>> > >> > > > > > > > >>> > >> -- Enrico > > > > > > > >>> > >> > > > > > > > >>> > >> > > > > > > > >>> > >> > > > > > > > > >>> > >> > > > > > > > > > >>> > >> > > -- Enrico > > > > > > > >>> > >> > > > > > > > > > >>> > >> > > > > > > > > > >>> > >> > > 2017-09-05 15:10 GMT+02:00 Jia Zhai < > > > > zhaiji...@gmail.com > > > > > >: > > > > > > > >>> > >> > > > > > > > > > >>> > >> > > > Hi all, > > > > > > > >>> > >> > > > > > > > > > > >>> > >> > > > I have just posted a proposal to remove > zookeeper > > > > > > dependency > > > > > > > >>> from > > > > > > > >>> > >> > > > bookkeeper client, to make bookkeeper client a > > thin > > > > > > client: > > > > > > > >>> > >> > > > > > > > > > > >>> > >> > > > https://cwiki.apache.org/ > > > > confluence/display/BOOKKEEPER/ > > > > > > > >>> > >> > > > BP-16%3A+remove+zookeeper+ > > > dependency+from+bookkeeper+ > > > > > > client > > > > > > > >>> > >> > > > > > > > > > > >>> > >> > > > > > > > > > > >>> > >> > > > BookKeeper uses zookeeper for service discovery > > > > > > (discovering > > > > > > > >>> the > > > > > > > >>> > >> > > available > > > > > > > >>> > >> > > > bookies in the cluster), metadata management > > > (storing > > > > > all > > > > > > > the > > > > > > > >>> > >> metadata > > > > > > > >>> > >> > > for > > > > > > > >>> > >> > > > ledgers). However it exposes the metadata > storage > > > > > directly > > > > > > > to > > > > > > > >>> the > > > > > > > >>> > >> > > clients, > > > > > > > >>> > >> > > > making bookkeeper client a very thick client. It > > > also > > > > > > > exposes > > > > > > > >>> some > > > > > > > >>> > >> > > > problems. > > > > > > > >>> > >> > > > > > > > > > > >>> > >> > > > This BP explores the possibility of eliminating > > > > > zookeeper > > > > > > > >>> > completely > > > > > > > >>> > >> > from > > > > > > > >>> > >> > > > client side, to produce a thin bookkeeper > client. > > > > > > > >>> > >> > > > > > > > > > > >>> > >> > > > I will send a patch as soon as we agree on the > > > > proposal. > > > > > > > >>> > >> > > > > > > > > > > >>> > >> > > > > > > > > > > >>> > >> > > > Thanks. > > > > > > > >>> > >> > > > > > > > > > > >>> > >> > > > -Jia > > > > > > > >>> > >> > > > > > > > > > > >>> > >> > > > > > > > > > >>> > >> > > > > > > > > >>> > >> > > > > > > > >>> > > -- > > > > > > > >>> > > > > > > > > > >>> > > > > > > > > > >>> > > -- Enrico Olivelli > > > > > > > >>> > > > > > > > > > >>> > -- > > > > > > > >>> > > > > > > > > >>> > > > > > > > > >>> > -- Enrico Olivelli > > > > > > > >>> > > > > > > > > >>> > > > > > > > >> > > > > > > > >> > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > -- > > > -- Enrico Olivelli > -- Jvrao --- First they ignore you, then they laugh at you, then they fight you, then you win. - Mahatma Gandhi
