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From:Johan De Meersman Date:October 28 2011 7:03am
Subject:Re: Master-Master -> duplicate entry
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----- Original Message -----
> From: "Marcus Bointon" <marcus@stripped>
> 
> Not at all. The point of redundancy (via replication or DRBD) is to
> maintain consistent and available data in the face of a server or
> replication failure.

That is the point of HA replication. You can also replicate for other purposes, read
scaling being one of them.

> like using RAID-0, but without the speedup! With writes to both

RAID-0 is striping, which provides no HA benefit whatsoever, and, indeed, worsens your
data availability: one lost disk kills your entire volume.

No, replication is more like RAID-1, if you really want to use those analogies: you now
have 2 copies of your data. Writes necessarily happen on both sides, so there is no
benefit there (except for HA of course), but you can now read from both sides,
effectively doubling your read capacity.

> there's no way of avoiding the split-brain scenario you describe and
> it's really hard to recover from (I've been there!).

Of course there is, but it requires your application to be aware of the scenario. As I
said earlier, this is not trivial, and most applications can't handle it. That does not
mean that there is no benefit for applications that do.

> There are some attempts to introduce a semi-synchronous replication
> system for MySQL that waits until transactions are replicated

Which is entirely irrelevant to this particular line of thinking, as it waits for the
transactions to be *logged*, not *applied* - there is still no two-phase commit and thus
no checking for conflicting updates.

-- 
Bier met grenadyn
Is als mosterd by den wyn
Sy die't drinkt, is eene kwezel
Hy die't drinkt, is ras een ezel
Thread
Master-Master -> duplicate entryRicardo Freitas26 Oct
  • Re: Master-Master -> duplicate entrynavid@navid.it26 Oct
    • Re: Master-Master -> duplicate entryRicardo26 Oct
  • Re: Master-Master -> duplicate entryRick James27 Oct
    • Re: Master-Master -> duplicate entryRicardo27 Oct
      • Re: Master-Master -> duplicate entryMarcus Bointon27 Oct
        • Re: Master-Master -> duplicate entryJohan De Meersman27 Oct
          • Re: Master-Master -> duplicate entryRicardo27 Oct
          • Re: Master-Master -> duplicate entryRicardo27 Oct
          • Re: Master-Master -> duplicate entryMarcus Bointon27 Oct
            • RE: Master-Master -> duplicate entryRicardo Freitas27 Oct
              • Re: Master-Master -> duplicate entrynavid@navid.it27 Oct
              • Re: Master-Master -> duplicate entryMarcus Bointon27 Oct
                • RE: Master-Master -> duplicate entryRicardo Freitas27 Oct
                  • Re: Master-Master -> duplicate entryMarcus Bointon27 Oct
                    • RE: Master-Master -> duplicate entryRicardo Freitas27 Oct
                    • Re: Master-Master -> duplicate entryRicardo27 Oct
                      • Re: Master-Master -> duplicate entryMarcus Bointon27 Oct
                        • RE: Master-Master -> duplicate entryRicardo Freitas27 Oct
                          • Re: Master-Master -> duplicate entryMarcus Bointon27 Oct
                            • Re: Master-Master -> duplicate entry (UUID lecture)Rick James27 Oct
            • Re: Master-Master -> duplicate entryJohan De Meersman28 Oct
        • Re: Master-Master -> duplicate entryRicardo27 Oct
  • Re: Master-Master -> duplicate entrylist27 Oct
    • RE: Master-Master -> duplicate entryRicardo Freitas27 Oct
      • Re: Master-Master -> duplicate entryMarcus Bointon27 Oct
        • Re: Master-Master -> duplicate entryMySQL)27 Oct
    • Re: Master-Master -> duplicate entryRick James27 Oct
      • Re: Master-Master -> duplicate entryMarcus Bointon27 Oct
        • Re: Master-Master -> duplicate entryRick James27 Oct
          • Re: Master-Master -> duplicate entryJohan De Meersman28 Oct
            • Re: Master-Master -> duplicate entryRick James28 Oct
              • Re: Master-Master -> duplicate entryMats Kindahl31 Oct
      • Re: Master-Master -> duplicate entrylist27 Oct