<!DOCTYPE html><htmllang='en'><head><title>Documentation | Taos Data</title><metaname='description'content='TDengine is an open-source big data platform for IoT. Along with a 10x faster time-series database, it provides caching, stream computing, message queuing, and other functionalities. It is designed and optimized for Internet of Things, Connected Cars, and Industrial IoT. Read the documentation for TDengine here to get started right away.'><metaname='keywords'content='TDengine, Big Data, Open Source, IoT, Connected Cars, Industrial IoT, time-series database, caching, stream computing, message queuing, IT infrastructure monitoring, application performance monitoring, Internet of Things,TAOS Data, Documentation, programming, coding, syntax, frequently asked questions, questions, faq'><metaname='title'content='Documentation | Taos Data'><metaproperty='og:site_name'content='Taos Data'/><metaproperty='og:title'content='Documentation | Taos Data'/><metaproperty='og:type'content='article'/><metaproperty='og:url'content='https://www.taosdata.com/en/documentation/faq/index.php'/><metaproperty='og:description'content='TDengine is an open-source big data platform for IoT. Along with a 10x faster time-series database, it provides caching, stream computing, message queuing, and other functionalities. It is designed and optimized for Internet of Things, Connected Cars, and Industrial IoT. Read the documentation for TDengine here to get started right away.'/><linkrel='canonical'href='https://www.taosdata.com/en/documentation/faq/index.php'/><script src='../lib/jquery-3.4.1.min.js'type='application/javascript'></script><linkhref='../lib/bootstrap.min.css'rel='stylesheet'><linkhref='../styles/base.min.css'rel='stylesheet'><linkrel='stylesheet'href='../lib/docs/taosdataprettify.css'><linkrel='stylesheet'href='../lib/docs/docs.css'><script src='../lib/docs/prettify.js'></script><script src='../lib/docs/prettyprint-sql.js'></script></head><body><script>$('#documentation-href').addClass('active')</script><divclass='container-fluid'><mainclass='content-wrapper'><sectionclass='documentation'><ahref='../index.html'>Back</a><h1>FAQ</h1>
<h4>1. When encoutered with the error "failed to connect to server", what can I do?</h4>
<h4>1. When encountered with the error "failed to connect to server", what can I do?</h4>
<p>The client may encounter connection errors. Please follow the steps below for troubleshooting:</p>
<ol>
<li>On the server side, execute <code>systemctl status taosd</code> to check the status of <em>taosd</em> service. If <em>taosd</em> is not running, start it and retry connecting.</li>
**MNODE的选择:** TDengine逻辑上有管理节点，但没有单独的执行代码，服务器侧只有一套执行代码taosd。那么哪个数据节点会是管理节点呢？这是系统自动决定的，无需任何人工干预。原则如下：一个数据节点启动时，会检查自己的End Point, 并与获取的mnode EP List进行比对，如果在其中，该数据节点认为自己应该启动mnode模块，成为mnode。如果自己的EP不在mnode EP List里，则不启动mnode模块。在系统的运行过程中，由于负载均衡、宕机等原因，mnode有可能迁移至新的dnode，但一切都是透明的，无需人工干预，配置参数的修改，是mnode自己根据资源做出的决定。
**重定向**：无论是dnode还是taosc，最先都是要发起与mnode的连接，但mnode是系统自动创建并维护的，因此对于用户来说，并不知道哪个dnode在运行mnode。TDengine只要求向系统中任何一个工作的dnode发起连接即可。因为任何一个正在运行的dnode，都维护有目前运行的mnode EP List。当收到一个来自新启动的dnode或taosc的连接请求，如果自己不是mnode，则将mnode EP List回复给对方，taosc或新启动的dnode收到这个list, 就重新尝试建立连接。当mnode EP List发生改变，通过节点之间的消息交互，各个数据节点就很快获取最新列表，并通知taosc。
if $system_content != @{"status":"succ","head":["name","created_time","ntables","vgroups","replica","quorum","days","keep0,keep1,keep(D)","cache(MB)","blocks","minrows","maxrows","wallevel","fsync","comp","cachelast","precision","update","status"],"data":[],"rows":0}@ then
if $system_content != @{"status":"succ","head":["name","created_time","ntables","vgroups","replica","quorum","days","keep0,keep1,keep(D)","cache(MB)","blocks","minrows","maxrows","wallevel","fsync","comp","cachelast","precision","update","status"],"column_meta":[["name",8,32],["created_time",9,8],["ntables",4,4],["vgroups",4,4],["replica",3,2],["quorum",3,2],["days",3,2],["keep0,keep1,keep(D)",8,24],["cache(MB)",4,4],["blocks",4,4],["minrows",4,4],["maxrows",4,4],["wallevel",2,1],["fsync",4,4],["comp",2,1],["cachelast",2,1],["precision",8,3],["update",2,1],["status",8,10]],"data":[],"rows":0}@ then
if $system_content != @{"status":"succ","head":["ts","speed"],"data":[["2017-12-25 21:28:41.022",1]],"rows":1}@ then
if $system_content != @{"status":"succ","head":["ts","speed"],"column_meta":[["ts",9,8],["speed",4,4]],"data":[["2017-12-25 21:28:41.022",1]],"rows":1}@ then
if $system_content != @{"status":"succ","head":["ts","speed"],"data":[["2017-12-25 21:28:41.022",1]],"rows":1}@ then
if $system_content != @{"status":"succ","head":["ts","speed"],"column_meta":[["ts",9,8],["speed",4,4]],"data":[["2017-12-25 21:28:41.022",1]],"rows":1}@ then
system_content curl -u root:taosdata -d 'select * from db.sys_cpu_d_bbb_lga_1_web01' 127.0.0.1:7111/rest/sql/
print $system_content
if $system_content != @{"status":"succ","head":["ts","value"],"data":[["2012-09-05 20:00:00.000",18.000000000]],"rows":1}@ then
if $system_content != @{"status":"succ","head":["ts","value"],"column_meta":[["ts",9,8],["value",7,8]],"data":[["2012-09-05 20:00:00.000",18.000000000]],"rows":1}@ then
if $system_content != @{"status":"succ","head":["ts","value"],"data":[["2012-09-05 20:00:00.000",18.000000000],["2012-09-05 20:00:05.000",18.000000000]],"rows":2}@ then
if $system_content != @{"status":"succ","head":["ts","value"],"column_meta":[["ts",9,8],["value",7,8]],"data":[["2012-09-05 20:00:00.000",18.000000000],["2012-09-05 20:00:05.000",18.000000000]],"rows":2}@ then
if $system_content != @{"status":"succ","head":["ts","value"],"data":[["2012-09-05 20:00:00.000",8.000000000],["2012-09-05 20:00:05.000",9.000000000]],"rows":2}@ then
if $system_content != @{"status":"succ","head":["ts","value"],"column_meta":[["ts",9,8],["value",7,8]],"data":[["2012-09-05 20:00:00.000",8.000000000],["2012-09-05 20:00:05.000",9.000000000]],"rows":2}@ then
